You take 2% off the top of what Stripe already takes. No different from DonorBox. You mention on your website that there are discounts for non-profits, but I assume it's not 100%.
The idea is that if Stripe offers it, they won't take anything off what they already take.
Warning: shameless plug.
It's a no-code shopping cart that works with Stripe. We offer Donations that work as you describe.
You could check it out - https://trolley.link
(you can email me direct at rg@trolley.link if you want to know anything more)