donation based payment for a non profit organization

Site URL: https://armtm.squarespace.com/

I'm working for a non profit organization called arm TM. We are fighting against child poverty.

Our current issue is that we want to change the way people can buy our t-shirts that are linked to a donation.
We want the customers to be able to choose their prize / their donation (attatched two images how we would like it to work).
We thought of different ways but they all have their problems. Is Squarespace really missing such a function?

Our first solution:
We could create different versions of the same Product via "Inventory -> Variants".
But then we would have different stocks for each of these Products. That wouldn't be possible – the are all part of the same stock.
Is there a way to link all those product variants to the same stock?

second solution:
We could add a donation block on the product page.
But we didn't find a way to make a donation mandatory. Is there a way to do that?

third solution:
We thought we could add the donation during the payment process in the cart. But we already found out that Squarespace doesn't support that.

Please help us. Thank you very much,

Leo