Or try this: get the paperwork, fill it in, then don't file, but send Prosper a copy showing that you're all prepared to file. The idea is Prosper would take you seriously and return your money to you without the cost of filing/serving.
The problem with this is that mailing forms around, or any form of postal mail communication costs so much money that each item you send consumes a significant fraction of the amount you are owed.
I went around and around with Shell Oil, who owed me $3.00, a few years ago. I sent one letter, but refused to send more. The only affordable communications mechanism in such a conflict is toll free phone numbers.
I would suggest calling daily. Be polite. Keep a record. Eventually you can try blogging about it.