There are a few reasons an order can be returned to sender but most often it is due to an undeliverable address. Other instances include the package being unclaimed at customs, refused/returned by your end customer or recipient being unavailable.
We would advise contacting your end customer to determine how they would like to proceed.
If you end customer would like the product to be reshipped, you can place a new order with an updated address. Please note you will be charged for the new order for orders that have been unclaimed at customs, returned by your customer, had an incorrect address originally provided or where the recipient was unavailable.