A large, well-known, chain retailer was packing large toys at the cashier station. The cashiers were instructed to ask very clearly, "Would you like to take this item as is, or would you like me to pack in in the large green bag?" The 'large green bag' came from a box labeled 'Trash Bags'. Oh, for Pete's sake! It it clean and does its job!
I disagree with the poster who said that trash bag means trash and there is an undercurrent or subconsionce feeling about using trash bags.
For me, it is just a nice large bag that will stay clean and do it's job or a grocery bag to be reused.
For the OP, I would definitely ask for the bags back. I have dificulty in understanding why this is a big deal, but it must be to you and MIL. Just go with the flow and please don't gather implications where none are truly intended. Just mention the bags you paid for, how handy they are, and how useful they will be in the long run.