Hi, artnick. Welcome to the Grumble!
I'll second what the guys said, about no easy fix. A paper conservator could probably do it, but especially since it's pastel, anyone else trying might cause more damage.
In my first year of framing, (ca. 1992), I accidentally damaged two things that were dry mounted with tissue, and my boss was able to (laboriously) remove both by
using Unseal. (Now, one such product is called Unstick, and should only be used with gloves and a cartridge mask). But, again, the combination of spray adhesive
on one side and pastel on the other make it more difficult. If the customer is willing to frame it, and if the shop has a CMC, perhaps you could cut a mat with a
rectangular window that has rounded corners? Or, if they don't want a mat and the denting is minimal, you could either add a fillet to the inside lip of a frame,
or use a flat moulding that's wide and deep enough, you could rip out some of the thickness to make a wider rabbet. (Or sell them a Picture Woods or Vermont
Hardwood frame, and they can do the rip out, for a fee). Or, you can try what Bruce posted while I was writing. If you try it, good luck!
Whatever they do, please be confident that you don't owe them anything. It's definitely a bummer, but they dropped it, not you.
Unless you're the shop owner and want to be super kind, it's okay to charge usual price for anything more.