Yes you can add a parcel rack on the rear, but since the relationship between frame and rear axle changes because of the suspension, your rack has to either account for this movement, or be completely above or below the suspension.
Downside is this one has few triangles and risks wobbling. A lot of the strength is lost by using rectangles in the design, and on a big bump the rack or load may hit the seatpost/saddle.
Downside of these is there is a lot of leverage, so the total capacity is low. This cantilevered beam rack also concentrates force on your seatpost, adding to the bike's perception of the rider's weight. Also, the bolts have to be super tight to stop the load waggling like a dog's tail as you ride.
You might consider a front rack, which goes over your front wheel. Downside, weight there affects steering, and impacts your visibility to the road surface. Also your photo shows V brakes, which might get in the way. Sometimes called a radonneuring rack.
You would probably benefit from a box/basket or milk crate on top of your racks. This lets you put items in without having to strap them down and crush them. Thermally lined shopping bags go inside the crate nicely.
Another option is a trailer behind your bike, which will allow bigger loads, better insulation, a less-stealable bike, and more visibility. Downside, more tyres on road means more drag, and 2-wheel trailers add width.
Last resort is to get another bike without rear suspension. This might be the best long-term plan and you can store the MTB for use off-road and all those MTB-like rides. Since uber-eats was mentioned, presumably you live urban with sealed roads. With a load on, you would not be riding off curbs/kerbs because that damages the product, so suspension is less needed.
Aside - never leave anything on your bike while away from it. Carry phones/money etc on your person at all times. Depending on your location, the bike could end up being stolen while you're elsewhere.