Lethargy and refusing food sound like the frog has succumbed to stress and has a bacterial septicemia - this is quite common in animals that go through the pet trade I'm afraid. Yours is not the first petco white's tree frog to be discussed in this context on the forum. An amphibian-savvy vet may be able to save the frog but aside from that it's highly unlikely to get better on its own. I doubt petco will do more than exchange it for you. I can't see them saving the exchanged frog though.