If you're talking about painting a bumper, you're probably not talking a LOT of money here (less than $1000, I'd guess?)
This is NOT worth a lawsuit. It will take much more of your time and money than you'll recover in court. It may be worth while taking them to small claims court. Small Claims court in NC will cost you a little under $150 to start with (once you pay the court filing fees, the summons, etc.), but if you win in small claims court, you can ask the magistrate to make the other side pay you back for court costs.
Based on the facts you've given, I'd say that the paint shop is the one most likely to bear the burden here. It is a standard industry practice for parts to come pre-primed (that is, it's not a "special order" in the legal sense). That the paint shop didn't prep the part to make sure the paint adhered to the primed surface falls back on the paint shop to correct unless they gave a specific disclaimer about their work on the bumper or for the price they gave you.
Review the warranty you have. If it's the paint that's peeling (and not the primer), the paint shop should have the burden to make it right.
I would first try to negotiate with the paint shop (would you be willing to split the difference?), but let them know that you're prepared to take this to small claims court.
Hope that helps.