Post Office refused to refund me for stamps return

Last week I mistakenly bought stamps which were more than I was supposed to buy. I only needed 20 for wedding invitations. I phoned the customer services and I was told that I can return them to the Post Office as long as it is still within 7 days. Today I went to the same Post Office and Nomusa who is a teller told me that I cannot return stamps even though they were in the same condition that they were. She told me, I should have returned them the same day that I bought them. I told her I phoned the customer services the same day and I was told to return them within 7 days. I am not happy with this inconvenience and inconsistency. The receipt itself even says that I can return the items within 7 days. Please explain to me, how you operate at the post office.