Tape is unfortunately uneconomical for regular people due to the drives costing so much, unless you get a used, older generation one.
How many GB’s do you mean? Maybe try some optical discs, BD-R at 25GB maybe.
Otherwise just rent some online storage. Hetzner Storage Box is cheap and Storage Share is only slightly less cheap, and has lots of sharing features (it’s really NextCloud).
How many of these donations are you processing per minute? Are they mostly cash, credit card, or what? If credit card, your payment processor likely has a category for donations, as well as different types of sale. These days they can usually email a receipt or print on on the payment terminal. I don’t see much self-hosted software in this.