Wow, I've been working on almost the exact same thing!
Except I'm charging a yearly fee instead of using the users own AWS account, this makes it easier for users without an AWS account and also means you don't have to worry about the crazy Glacier fee structure.
Try it out if you want, 1GB accounts are free at the moment:
Try it out if you want, 1GB accounts are free at the moment:
https://www.tidy.io/