If you're seeing the error Missing or invalid shipper number while trying to use your recently connected UPS account, you will need to sign in to UPS.com using the same UPS credentials you use in SKULabs. From there click the "My UPS" tab, then "Manage Accounts".

Ensure your account number is listed here. If you do not see your account number listed, click "Add a UPS Account" to link your account number to your UPS account.

Source: https://www.ups.com/at/en/help-center/sri/apishippingerror.page

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