How to purchase a phone through amazonwireless or overstock?

On amazonwireless and overstock.com you can purchase tmobile phones either as a new tmobile customer or an existing customer. I selected that I am an exsisting customer with an individual contract and yet I'm still asked to purchase a plan. I wasn't sure if this would have a bad effect on my tmobile account at all. I tried several times through customer service to try to find out why I have to purchase a plan if I already have one. So far, no one has been helpful.