I guess I never saw it as much of a burden myself. It's her cat, her mom was doing a favor. The favor fell flat, but ultimately it would still be the OP's responsibility to get cat litter for her own cat.
I used the word "burden" because there are only a couple of outcomes possible prior to her mom volunteering to return it (a favor gone wrong can cause additional burdens/inconveniences/thesaurus options for the one in need of a favor):
1) You graciously accept the many bags of litter, because you don't want mom to have to go out of her way yet again. Pay her back what you owe her. You still need to go and buy litter and bring it back by bicycle, plus now have several more bags that you somehow have to return to the store by bike.
2) You graciously accept the many bags of litter, because you don't want mom to have to go out of her way yet again. Pay her back what you owe her. You have to find someplace to store litter until you can find away to get rid of it and hope you caan recoup the cost of what you reimbursed to mom, and then you have to spend more money buying the right litter.
3) You graciously thank mom and ask her to return it.
In cases #1 or #2, the outcome of the "favor" had the opposite effect of its well-meaning intentions. Certainly mom was trying to do the OP a favor to help her, and she can't be faulted for that. I don't think Mom was rude at all, but I can sympathize with the OP that it's really frustrating that her mom checked with her only after
she'd paid for it and loaded it into her trunk.
To be fair to Mom, it's unclear from the OP whether Mom was carrying her phone with her at the time of purchase and photographed the litter in the store's parking lot, or if the soonest she could check with the OP was when she got home. If the latter, then she was
checking with the OP before going all the way to her place to drop it off.
The OP did say her mom lived in a different area. So if she didn't have her phone with her (and didn't feel comfortable borrowing the pet store's phone), it may have been more convenient for Mom to buy the litter, check with the OP and if everything was good, then great — off to OP's house! If it wasn't, then her plan was to return to the store.