Donation based payments with minimum amount

Hi all, I wondered if anyone can help.

I haven't yet got a commerce site but I am wanting to know if there is a functionality that would allow me to put a minimum purchase price for an item with an option to then donate more should someone want to?

a bit like a minimum bid.

Or whether the commerce only lets you create a fixed price for the item.

Is anyone able to show me the backend of their commerce site? So I can see how a payment system works on square space.

Thank you everyone.