It depends on what the client has asked for really.
You can only sent a return package if you received a package in the first place.
However, it is worth noting the RPPX will only contain the translated SDLXLIFF file- no other resources are normally in the package.
Generally most clients will just want the translated SDLXLIFF file and /orTarget file in the original format.
Some may however require the updated project TM.
Best just to speak to your client to check