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I use my bike to run errands on the weekends - stopping for soap and shampoo at the drug-store, a loaf a bread at the grocery store, stopping for lunch, etc. The issue is I have to haul my previous purchases around with me in each store.
Is there some kind of "trunk" that I can add to my bike in which I can secure/lock/protect my purchases? I've seen such things on scooters, but is there something for a bike?
Locking trunk bags are available, but I really don't think they're the best solution. There aren't many of them, which means you don't have much of a choice of features. By definition, these are attached permanently to your rear rack, and if someone steals the bicycle, the locking trunk rack is theirs.
You have many options; here are the common types of cargo systems for bicycles.
Baskets:
You have many options for carrying cargo on a bike. For the kinds of items you describe, by far the most convenient carrying system is a wire basket. You can get ones that install on your front handlebars or on your rear rack, depending on personal preference or what kind of bike you have (and if you have racks already). You can also get a milk crate or a box and attach it to your rear rack.
Since a basket is attached to the bike, you don't need to worry about unhooking it from the bike when you lock the bike up. A cargo net is advisable with a basket, so that items stay put. However, they do make the bike a little bit more unwieldy and can rattle a bit. Also, small items can fall out, and contents are exposed to the elements.
Security: None, unless you carry everything along. (Not what you're looking for.)
A basket on a commuter bike:
Backpacks:
The next most convenient way to carry small amounts of cargo is with a simple backpack. The bike is unencumbered with racks or baskets or panniers, but your weight is up high (unbalancing you a bit) and your back can get sweaty. However, a backpack is useful for many things other than bike riding.
This is the ultimate in security, but has the disadvantage that you need to carry it around.
Panniers:
These are bags designed to hook to the side of a bike's rear rack. Panniers are more suitable for longer trips, and can be expensive and a little fiddly. You can't really leave them on the bike when locking it up. These would be overkill for your situation, unless you ride in the rain a lot and need to keep things dry.
Security is nonexistent. If somebody wants these, they're theirs.
A bike with front and rear panniers:
Trunk rack bags:
A trunk bag sounds like the sort of thing you were thinking of. It's a small bag, about the length and width of a bike's rear rack, and generallyas high as it is wide. I only recommend one of these if you already have a rack. Capacity is smaller than most backpacks. They're if you don't want a basket or a backpack. Some models are fiddly to attach or detach.
Locking versions of these are available but they're heavy and much less flexible than their counterparts for motorcycles. Part of the problem is that a bicycle is much lighter than a scooter or motorcycle. Also, a rack can be removed from a bike by anyone with a 5mm allen key.
Security: Good.
Trunk rack bag on a folding bike's rear rack:
Trailers:
Cargo trailers are available for bikes, and some can even be used as shopping carts. These are better for large grocery runs, and it sounds like you don't need a trailer at this point in time. Enclosed trailers are well-suited for riding in the rain.
Security: Very little, similar to panniers. Although you could get a trailer that converts into a shopping cart and take it with you, like this one. But it'd be overkill for small errands.
A trailer attached to a bike:
In summary, different kinds of carriers are good for different kinds of errands, but I recommend either a backpack in your situation. A locking trunk bag is only as secure as the rack on your bike, but may be useful if all you want is a deterrent.
You can get hard shell bicycle storage, but they're usually heavy, expensive and fragile. These panniers on Amazon are typical. Yes, they have a rigid shell and look sort of solid, but in practice the sort of knife that will cut soft-wall panniers will also cut these. Partly because decent panniers are made of quite strong cloth, and partly because really solid panniers end up weighing more than most people are willing to carry. The motorbike "Metal Mule" systems weigh 5 or 6 kg just for the boxes.
For low-value stuff like you're talking about I suggest just using normal panniers and locking those to your bike with a cheap padlock. Make it nice and visible to discourage casual theft - someone can still rummage through them and get stuff out, but they can't just grab the whole pannier.
The next step would be a metal luggage net that you can put over the panniers and lock to the bike. It's still pretty lightweight and makes it really annoying to get things out of the panniers.
For real security I think you'd need to make something yourself. Buy a box that can be secured and bolt it to your rear rack, probably with a bit of wood between box and rack for reinforcement. Where I live (Australia) there are a lot of those aluminium boxes you see on tradesmen's utes/trucks, and a custom smaller size one for your bike would probably cost about the same as the carbon fibre panniers above. My custom toolbox for home cost me about $200 but it's nearly ten times as big as you'd want to put on a bicycle, a smaller custom one should be under $100.
Go lo-tech and bolt a crate to the top of a 4-point pannier rack, use wing-nuts to secure it with the wing-nuts inside the box so you can take the contraption off quite easily.
Then put whatever you want inside and secure with bungee cords, bits of ugly string and a bit of unobtrusive tarpaulin. Make it look like you carry your dog in the back, totally agricultural. Anyone wanting to steal your bread and shampoo and soap will need lots of hands or a bag of their own. They would also have to be quick as 'it appears that you will be back in a matter of seconds' and anyone that does bother to steal your bread probably needs it more than you.
Posh solutions have been marketed in the past, however, they attract thieves. But, if that is what you want, put a motorcycle helmet box on the back - with wing-nuts inside - and that will blend into the environs really well as people walk past those all the time without wanting to pop them open. You can probably get a second hand one on eBay and that might blend in even more, plus you won't be so concerned about drilling holes in the bottom of it to adapt it to your 4-point rear rack.
Something like this grocery bike may give you some good ideas. Basically what might help is a trailer you can detach and bring with you into the store and push around like a cart. That way, you don't have to "carry" anything around the store, and you don't have to worry about locking it up. The link I posted is to a custom bike, but I imagine that trailers must exist that you can detach and push around. Something like this trailer may do the job. Haven't used one before, but looks like it has the right features. Might look a little weird with a trailer made for kids, without the kids.
Even if you use a trailer, you would want to lock the trailer frame with a cable through your chainstay and seatstay. I have seen pictures of Pelican boxes bolted to to the sides of racks.
Set and setting (as Timothy Leary has put it) is important for transporting valuables. I've parked in the afternoons at my grocery store and found people come up to their bikes and discover their backpacks stolen...but I don't discover that situation early in the mornings. Your best protection might be avoiding times or localities that attract thieves.
If you have friends that can accompany you on a trip, this is also an effective method of preventing theft. I have gone shopping with my family and often someone stays with the kids and watches the bikes. Or, if we're toting a trailer that doesn't lock to the bike frame, someone will wait outside.
Other tactics, even though these can inconvenient and possibly more expensive, would be to a) load your panniers into your shopping cart to keep an eye on them, b) plan your trips so that you don't chain a enriching stop before a risky second destination, or c) have the item delivered.
