payment to united Cash Loans

I have a loan from United Cash Loans for $300. I have so far paid one finance charge of $90, and closed out my bank account before they could take out anymore after getting educated on payday loans through this forum. Beginning yesterday they started calling me saying that I owe them $390 and I could settle in full by using a debit card or a prepaid debit card. Now I realize that technically I only owe them $210, but I recently was put into a position where I can pay the loan off in full. I would be willing to pay the $390 just to be rid of them once and for all, as I have read absolute horror stories about them. My only concern is if I get a prepaid card( I would NEVER give them a debit card number) and I pay this, how can I be sure they will actually credit me and not come back saying they never got payment. They won't do anything in writing, and don't take mail in payments or money gram. Will the pre paid card company be able to give me some sort of record that a transaction was ever made. Please, I just want to get this over with. Im supposed to pay tomorrow, so a swift response would be appreciated. Thank you.