For international sellers without a default US return address

• Automated returnless refund: If you are fulfilling customer orders in the US from outside the US and a default US return address is not provided, Amazon will automatically issue the customer a returnless refund for eligible returns equal to or under 25 USD without requiring the item to be returned.
• Returnless refund: For return items above 25 USD, you may offer a full refund without requesting the item to be returned.
• Provide prepaid international return shipping: Provide your own prepaid return mailing label for a return request from the stores in which you are selling. For example, if you are selling on Amazon.com and shipping from China, you will provide the customer with a return label from the United States to your location in China. We recommend that prepaid return shipping labels include tracking.
• Enroll with a service provider: Our international returns provider enrolled in the Returns Provider program or a provider on the Solution Provider Network can help if you do not have a local return address. Using the US domestic return address established as part of your use of a service provider, you will automatically be enrolled into the Amazon Prepaid Return Label program.
• Automated Refunds: If you do not offer any of the above return methods within five days of a return request, Amazon may refund the customer on your behalf and charge the amount to your seller account.

Amazon has also launched a Returns Provider Program for international sellers that you may want to look into.

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