I have never done a sheet metal repair, nor did I stay at a Holiday Inn Express last night, but if a factory-built, non-damaged elevator doesn't exhibit the same "oil canning" properties as the repaired elevator, I would suspect the repair is not up to snuff. Perhaps a call to your local FSDO would be in order, as you suspect you've been sold a repair that doesn't meet airworthiness standards? If the FSDO won't play, call a Designated Airworthiness Representative (DAR). He will charge you, but his word is like that of the Oracle, so the shop and the FAA would have to respond. Just the threat of getting a DAR involved may cause the shop to reconsider it's position regarding the repair. Or, on the other hand, you may find out that "oil-canning" is perfectly acceptable. Either way, you'll know. Even if it is acceptable, I wouldn't want to hear it every time I went flying!
Here is an engineering forum where actual aircraft engineers discuss oil-canning. According to what I see there, no oil-canning is permitted as it negatively affects the flight control:
https://www.eng-tips.com/viewthread.cfm?qid=248639
There are many mentions of oil-canning on the Vans RV builder's forum, and the solutions range from re-skinning to "don't worry about it". So a definitive answer may be hard to find without a DAR or DER getting involved...