Sending an item for repair out of the country - how do you avoid custom duties when it is returned

[longwood50]

I have a hearing aid that needs to go to the USA for repair.  When it is returned, how does the shipper identify it as a repair rather than a purchase subject to customs duty.  Some have suggested that I need to identify it when shipping it out of Thailand.  

[JamesE]

On the customs form there is a check box for "Returned Goods". In the description line the repair station could describe it as a return of repaired item or something. https://about.usps.com/forms/ps2976r.pdf