Nedbank Credit Card

I have made an arrangement on my credit card earlier 31Jan 2020 for payment of the amount of R1600.00 on the 26th of February. I have recently received a sms stating that a letter of demand was posted and that i should collect same. I am upset due to the fact that arrangement was made and that i have kept my arrangement with Nedbank. I have made a commitment to paying thise amount on the 26th of February.