I have a few shops in my place at rabbithole and for when people donate and all the items are removed I do not want the shop to close.
So what I am supposing could happen is a donation button right under the slider for setting the amount you are required to pay.
When you tap the donation button it will no longer be required that an item should go in there, but it would want you to set the price of the donations you are planning to do.
This would go very good for people who wish to put donation shops in their housing/shop places. As it would relief the stress of refilling the shop after (ex: 99 items) are all bought out.