Which retail channel allows USPS customers to order postal products through the mail?

The option "Stamps by mail" is the correct answer because it specifically refers to a service provided by the USPS that enables customers to order postal products, such as stamps and mailing supplies, directly through the mail. This service is designed for those who prefer to receive their postal items without visiting a physical location, thereby offering convenience and accessibility to customers who may have difficulty accessing a post office.

The online store, mobile app, and in-store options provide different channels for purchasing postal products, but they do not encompass the specific mail-order aspect that "Stamps by mail" offers. The online store and mobile app both require internet access and a digital transaction, while the in-store option necessitates a physical visit to a USPS location. In contrast, "Stamps by mail" effectively bridges the gap for those who are more comfortable with traditional mail services.