All shopping agents support returns. But the sellers also have to accept them, which most sellers do.
Usually there's a time limit for free returns, usually 7 days.
Handling exchanges can be difficult, and usually the procedure that's encouraged by the agent and easier for you is returning the item and ordering it again.
Some agents have limits on how many items you're allowed to return per month, after which you have to pay a fraction of a dollar for a return, to discourage abuse.