This manual links to the Stamps.com guidelines for inputting IOSS number to the Stamps.com solutions.
Follow this link to Stamps.com: https://www.stamps.com/article/how-to-input-vat-ioss-numbers-into-customs-forms/
Follow this link to Stamps.com: https://www.stamps.com/article/how-to-input-vat-ioss-numbers-into-customs-forms/