It's probably due to RFS (Refund First Scan) in which Amazon refunds the buyer upon the first scan of the return package. This usually happens with packages valued at under $100, but it doesn't always happen. If it isn't a refund from RFS, then it likely means the buyer complained to Amazon about something and Amazon refunded out of your funds. This is much more likely to happen if you are signed up for CSBA (Customer Service By Amazon). If you are not signed up for CSBA and they refunded on your behalf, you can file a SAFE-T claim to attempt to get reimbursed. If you are signed up for CSBA, you can try the SAFE-T claim, but your chances are probably less.