OLD NAVY and their crazy exchange policy

A couple of weeks ago I went to old navy to purchase some school uniform pants. The only size they had left was a size 6. After getting home and trying them on they were a bit big. I could not find my receipt so I had to do an exchange. They still did not have the size 5 so they had to send me a merchandise certificate in the mail which took 7 days. At the time I made the exchange they had the pants online and the girl told me I would have to go to the store to use the merchandise certificate to place the order from their online store. Keep all your receipts from old navy before you get screwed like I am. Bad company policy. They don't even work with you to try and help you.