Sounds sleazy on CJ's part. They should have told you about the shipping damage when you called to pay for the repairs. Also, if they received the pre-amp and the exterior was seriously damaged, why did they not call you before doing the repair?? Did they expect you to place back on your audio rack the repaired pre-amp in its twisted condition.
I can only conclude that after they received it, and IF IT was not showing damage, that this damaged occurred after they received it. Could be something fell on the pre-amp while it awaited repair,or while on the service bench.
You need to recoup the repairs charges and the USPS transit insurance money. Just make sure you have all documentation and photos of the damaged pre-amp before making the claim. USPS is a foundering bureaucracy