Pro-tip: don't get the manager involved
Another pro-tip: the written agreement on the receipt holds no worth.
I bought some sunglasses because I lost mine in the ocean. After walking around town for 2 hours with my new glasses I noticed they gave me a headache. I looked at my receipt to see the return policy. It said something to the effect of: may exchange for in-store credit within 7 days of buying the item. When we returned to the store counter we asked if we could just return them, the nice cashier said "only for in-store credit" so we asked to consult with a manager.
She echoed the same rule, which is in line with the receipt's written policy. Then she went on to say that since the product was used she wouldn't exchange them, they are unsellable because they were used! We went back and forth about the sunglasses being in perfect condition just needing cleaning. She was sarcastic and rude, continually comparing the sunglasses to returning sandals, which this is very clearly an incomparable product.
My suggestion is that if your policy is not to exchange used products, you need to have that in writing on the receipt and or say so when making a sale.
If you don't have that in writing, you are bound to the agreement you do have. You cannot make up rules based on how you feel that day or how much you like or dislike your customers. Boo on you, miss manager. read more