25 dollar 2 months after I paid my final bill

I stopped service on February 25. I paid my final balance in April and my balance was zero at that point. Couple months later I get bill for 25 dollars. I called to ask what fee is for , only to get different explanation. At the end of call I told them that I called and paid my final balance in April. If there were any late charges are anything that has occurred between February 15 and April 18, it would be part of balance . They agreed and they said they will give me credit for 25 and I don’t have to worry about it. Next call I get is from collection company trying to collect on 25 dollars. As it is easier to pay 25 than go trough this, at this point it doesn’t seem paying fees is the end of it. I thought paying some fees before was end but apparently it’s never ending cycle