First lost A-Z claim in a decade

I guess I should count myself lucky. Today I received a notice an A-Z had been decided against me. It’s the first time. It was a buyer in Norway who claimed I didn’t enter Amazon’s VOEC number so they got dinged twice for tax.

I use Stamps to ship my international Amazon packages. I confirmed thru their site I entered the VOEC correctly in the “IOSS” space on the customs form like I always do. I’ve never had a customer get dinged for taxes twice so I don’t know if this is a one/off or if I need to change how I’m doing things in the future.

Do you sellers who ship internationally just rely on putting the VOEC/VAT code in the customs form, or do you also write it on the box? I certainly want to make sure this doesn’t happen to future buyers.