Receipts make it a no brainer, however, if it is a branded product or on their "special discount" flyers there is no problem. Problems occur when item was not purchased at that store or store no longer carries item.
In most cases yes, you need the receipt.
It makes it easier to return, but if it is their brand and unopened they have exchanged in the past for me.
Not if you have an account
Call them and ask
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