Follow this guide to add your UPS shipper number (account number) to your ups.com profile. This should solve the UPS error `120100: Missing or invalid shipper number error` when using UPS shipping API or Postmen labels API.
Add UPS shipper number to ups.com
- Log in to ups.com (Be sure to use the same User ID and Password that is in your Shipping API)
- Select Account Summary from the My UPS tab rollover menu.
- Verify your account is listed; if not, select the Add a UPS Account link
- Follow the instructions to add the account