I returned one pair to Costco. I called and told them I wanted to make the return. When I arrived, they had all the paperwork ready and processed the return with absolutely no problem. They told me it would probably be more difficult to return the new aids I ordered. Some customers have reported being black-listed for replacing 2 sets.
Costco are top-notch retailers. They know very well that they have to meet their written commitments and that doing so with a smile is the best way to do it. Have no fear of using their return policy.
My reco is to focus on the aids. If they make new aids work for you, keep 'em. If they don’t, return 'em.
BTW, custom molds and accessories are returnable, but I don’t know how long the return period is.