What shipping provider do we use for printing mailing labels and verifying shipping addresses?

Answer: A — Shipstation

Shipstation is the correct answer because it is specifically designed for printing mailing labels and verifying shipping addresses, while the other options are general shipping providers that offer a variety of services. UPS, FedEx, and USPS do offer some label and address verification services, but they are not their main focus and may not be as efficient or user-friendly as Shipstation.