Where do US booksellers put the IOSS # on the customs form if the order is going to a EU country?

United States senders should provide the IOSS VAT Identification Number in the “Importers Reference” fields when sending to an EU country. This does not apply with consolidated shipping or via DHL.

For additional information, please visit https://faq.usps.com/s/article/Import-One-Stop-Shop-IOSS