Hello Rik,

please see the following links for information about the TIN and other taxation related topics:
https://help.spreadshirt.com/hc/en-gb/articles/207194399-Taxation-of-Your-Earnings
https://help.spreadshirt.com/hc/en-gb/articles/212640345-Withholding-Tax-on-Design-Price-Earnings
https://help.spreadshirt.com/hc/en-gb/articles/207905515-Payment-of-Your-Earnings

You can also use our Forum for additional information or discuss topics with other designers.

Best regards,
Sandra