They should be able to look up your purchase by your member number
If you're a REI member you have one year to return with or without a receipt. If you are not a member, then you need that receipt.
Perhaps, if you kept the tag with the bar code or some other identifying marker. Also, if you have an REI account, which I recommend, they will return it no receipt, no questions. Take it in and try, they're the best when it comes to returns.
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