Bridge of contract

I have a problem with metropolitan i received a letter ealier in 2021 that my premium for my daughters educational plan it will increase on the 01 January 2022 but when they go and debit on my bank account on the 31 December 2021 the debit order was returned because i had R430 and the premium before the increase was R403.00, so according to my understanding the have bridge our contract because the increase is supposed to start in January 2022 not December 2021. I decide to make manual payment using my bank account on the 31 december 2021 amount of R443.00 but today on the 05 January 2022 i receive sms saying i need to pay R442.83.Please tell me what happen to the payment i made on the 31 december 2021.