paypal has a free developer sandbox. also if you have a credit card and so does a friend - you both can sign up and then revert the transactions (do trans less than $5 anyway ...) paypal allows no-fees reverts upto 30 days from the day of transaction.
the paypal site has a samples of the sandbox etc. making an account is free.
you would want to do a few real tranactions (and cancel them later ) when you do go on prod