Hesitating because a shop installed the "right amount" of sealant belies the misconception that sealant volume is anything like an exact science. It's actually quite the opposite. Different tire casings can take very different amounts on initial installation before they develop a coating sufficient to hold air effectively. Beyond that, there's a choice best made by the cyclist on how much free liquid sealant you want sloshing around inside. More gives you faster and bigger puncture fill and/or a longer maintenance interval, but at the expense of weight.
Air pressure going low over the course of a ride or a day is usually caused by needing more sealant. In your situation, if you're trying to be kind of weight-minimal and presuming you've got tires in the 25-28mm range, I would open the valves up and start by injecting around 1oz/30ml each, with the goal of getting a bit better coating and then having some modest free liquid on top of that. If you want to go a little longer without having to mess with it, double that volume.
Shops do their best to choose an appropriate sealant amount for any given situation (install, maintenance refill, etc) but there's guesswork involved. What sometimes goes unsaid, and often does in marketing and product info, is that to be an effective tubeless user you need to develop your own rhythm with sealant fill quantities and timing, based on your own equipment, conditions, and weight vs hassle priorities. Also, in my experience those o-rings make a pretty marginal difference either way. They're not the primary seal and things have already gone way wrong if you're losing air out of them. I don't know why they left it on if instructions said otherwise, but it's possible they had experience that contradicted the instructions. Try it both ways and see if you can tell a difference, or maybe check back in with them to see if they know something specific to those rims.