Contest Submission Button that Charges Participants

I have a writing-contest site in the works. I’m looking to incorporate a button (for entree into the contest) that requires participants to pay a small fee (probably about 2 dollars). I’ll also want a similar button for premium submissions (that will also get constructive criticism). If you have any ideas about how to incorporate such a button (whether it be a plugin or a widget, etc), please let me know.