Perhaps they will not pay to a charity whose premise they disagree with? Weirder things happen.
There are no Blocks against charities holding Paypal accounts or any other form of payment processing.
Politely inform them that you are a registered charity (if you are) even giving your registration number if you want, but that Paypal is the only form of payment that you can accept.
Then ask if they wish to cancel the purchase.
If they agree, cancel, and add them to your Blocked Bidder List as time-wasters.
Not every sale is worth having. This is too high-maintenance, you will waste time on her and time is money.