Help!! United Cash Loans demands for more

My loan with united cash was for 300 on the 15th of march. An interest of $90 went through on the 31st of march. Next interest on 16th April did not go through. On the 24th of April i called and spoke to a rep in their collections department who told me my balance was $510. At the time i was way to flustered to calculate because I had 7 of them going at me. I told him i didn't have any money. He offered that I pay $420 now or make 2 payments of 255 each. Truly I was mixed up and thought the actual loan I had was $350. Anyway when I sat back to look into all the accounts and attempt repayment plans, I discovered all I loaned was $300 and my balance therefore should be $420 = 300 +90 +30(NSF). I paid $200 and called to clear the amount, the rep i initially spoke with was absent and the agent there tried to put me through to a supervisor but he told me that they were in a meeting. the next time I called back the first agent I had spoken with tried to give a reason for the extra $90 and his reason was i did not call them before the due date of 16th. i have never heard anything like that before. Again I asked to speak witha a supervisor and still none was available. in the meantime I paid in the remaining $220 and called back. finally i spoke to the supervisor who is insisting that i have to pay the $90. What can i do? Please anyone with experience, please help me. I am still struggling to pay down some of these loans.