Bad payment allocation and service

After settling my account in full and using the correct references as instructed when requesting the settlement amount. 13 days later with my money in their bank Woolworths has since handed me over to a collection agency and have charged me interest on an amount outstanding that have been paid already. The worst service I have ever received after being a client for so long. I have followed up numerous times and their consultants cut my call and take ages to resolve. Does it really take this long to allocate a payment????