How Do I Avoid Return Shipping Costs?

So I ordered a ton of shoes from Amazon, anticipating that I'd be returning some of them after I tried them on at home then decide if I liked them. But all the while I thought they offered free return shipping. It's only free if you say "Defective/Does not work properly" or anything involving an error on their part. My question is what if once they receive the item and see that it's not defective, they don't reimburse me for shipping? It's gonna cost like $7 per shoe and I ordered 9 pairs of shoes. Ugh.