How do returns work? Who pays return shipping for different return reasons?

We sell records on Amazon. FBM. I understand when the item is returned for a defect we lose the price of outgoing shipping and pay the cost of return shipping so suffer a net loss. (Is there a way to stop even getting these returned?)

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We also get returns for “no longer needed” or “better price available”. When I check the order page there is often a balance of $4.xx left over from the order. Is this Amazon giving us the money for the outgoing shipping? Where does the money for the return shipping come from? I don’t find the Amazon invoicing/order page very intuitive/descriptive.

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I’m just trying to figure how much/if we’re losing on these customer returns where the customer changed their mind.

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Thanks for any help/resources to read