As it turns out, we actually cannot share the UPS thru ShipStation account's shipper number. This is simply a limitation of that UPS thru ShipStation account, platform wide.
So, to utilize the type of shipping you described, you would want to have your own direct UPS account and share that account's shipper number.
Davis, are you suggesting we all share the same UPS By ShipStation account number? All UPS tracking numbers start with 1Z & the next 6 numbers are the account number. Based on what your saying every UPS tracking number through UPS by Shipstation has the same first 8 characters?
The reason the account cannot be shared with third parties is that the Terms and Conditions for ShipStation's UPS offering expressly prohibit doing so without your securing prior written consent from UPS. Here is a copy of the Ts and Cs that are signed when granted access to the UPS from ShipStation integration for your review.