My understanding is that in using the term “sidewalls of the bed” the OP is referring to the inside walls rather than the outside. That doesn’t imply that they shouldn’t have been painted, but I think we have a misunderstanding here. If I’m incorrect I hope the OP will correct me.
OK, now I am confused. At first I thought every thing had been painted but the inside on the truck bed. To me that means the floor and the inside of the outside panels.
As for bed liner cost Google or the old fashion way, the yellow pages and get estimates because we don’t know where you are.
If someone agreed to paint my truck for X number of dollars , I’d expect the whole outside of the truck to be painted unless different arrangements were agreed to .
I had the bed sprayed with rhino liner. It seals the bed seams and things don’t slide around in the bed. They can tint it to any color you like. I don’t remember the price though.
You can buy the roll on DIY bed liner for about $50 but just the floor not the sides. The inside sides should be painted. If you use the drop in bed liner make sure its secured. I’ve seen them fly off into the ditch before.
I guess I would have expected that the inside of the bed would be painted but that’s the kind of detail you go over when making the deal. Same as painting the inside door sills. Normally they would not be painted unless changing colors and then only at additional cost and a separate line item because its a pain in the neck. When I had mine painted years ago, they gave me some left-over paint to do the insides of the doors if I wanted. I wonder where that yellow paint went from 1966?
I too would have expected the entire inside of the bed, floor and walls, to have been painted unless otherwise specified. But none of us knows what was said. I still recommend that the OP do nothing until talking with the shop owner. This may have been just a misunderstanding between the shop owner and the painter.
I would be interesting in hearing how the discussion went. I do care.