$20 is really gonna break the bank
It's not about an individual 20 dollars.
Let's say a pledge of $25 apiece is made by 30 individuals. Rusty then books both his flight and hotel room on the expectation of receiving $750 (less Paypal fees) in fan sponsorship. Then, for whatever reason, Paypal decides to freeze the account.
For those who are unaware, when this happens - and it happens quite often - Paypal does *not* automatically return the funds to the senders. It usually takes a few weeks to resolve issues on frozen accounts, but it can take months. The only remedy is to have paid by credit card, as you can then reverse the charge on your credit card and stick them with a chargeback.
If something like this should happen (a possibility you have to take into consideration), then who would be liable for honoring the $750 pledge to Rusty?