Amazon FBM returns

Hello, I am currently selling on Amazon as FBM and I have a questions about returns if anyone can answer. I currently do not offer free returns for any of my products. For some of the returns, the shipping fee is deducted and I don’t have to offer the full refund (since the shipping cost came out of my pocket). But sometimes, Amazon doesn’t deduct the shipping cost of the return from the customer.

Do I have to offer full refund even though I don’t offer free returns or can I simply deduct the shipping label cost from the total refunds?

Also, I am selling in the clothing category and the return reason for most are “doesn’t fit”.

Thanks 🙂