Using a customer's UPS account number in QuickBooks Desktop (QBDT) Shipping Manager isn't advisable, Vincenzo.

QBDT requires users to have their own shipping accounts for processing shipments. This is because when you first access the QuickBooks Shipping Manager and select UPS, you'll get prompted to configure the service. Then, you enter the "Ship From" address and UPS account number only once. 

So, if you don't have an account yet and use your customer's UPS account number, then this will be saved and used for your future shipments. 

For more details about setting up and utilizing the new Shipping Manager, refer to this article: New Shipping Manager in QBDT.

You may also check the FAQs section of this resource to address your UPS-specific questions: Intuit QuickBooks Shipping Manager.

Comment below if you have further queries.