Going back to AT&T

 Hi, so I bought an att iPhone 6 in the apple store without contract. I had a monthy plan with att, but some months ago, I change to T Mobile. However, now I want to go back with att. When I left att, I owed 40 dlls for a late payment. If I go back, do I have to pay an extra fee? or those 40 dlls?
thanks.