In the US (not sure what it costs for international), registered mail costs nearly $10, plus postage. Items shipped by registered mail remain under lock and key throughout the delivery process.

You may be thinking of certified mail, which is more of a tracking and delivery receipt process. That costs $2.65 without a return receipt, and $4.80 with a return receipt in the US. Plus postage.

You are fortunate that the seller was willing to offer a refund.