RAISE INTEREST ON A FULLY SETTLEMENT ACCOUNT / BAD SERVICE

I received a mail 01 October with an outstanding balance I paid this amount by the following morning before 8am. Interest was raised on the account the following day WHY and nobody is able to assist. Why am I paying interest when the very next day I send proof of payment for full outstanding balance. I have found it extremely difficult to even correspond with anyone they have staff dealing with accounts who don't even seem to understand things. Why am a being called by staff who do not seem to understand the basics of accounts? And I keep getting told the supervisor will call me I am still waiting I just want a paid up letter but this is disgusting service.