USPS

Sources: Amazon, the USPS' top customer, providing $6B+ in 2025 revenue, is considering ending its deal at the end of 2026 and expanding its US delivery network

Amazon has long been the Postal Service's top customer, providing more than $6 billion in annual revenue to the agency in 2025.

Docs: Amazon generated ~$3.9B in revenue and $1.6B in profit for USPS during FY2019; USPS delivered 1.54B packages for Amazon, or 30% of Amazon's 2019 volume

Lob, an online service for businesses sending snail mail to customers, raises $20M Series B from Y Combinator's Continuity Fund

The postage business may seem antiquated in the year 2017, but physical mail is still kicking.  In fact, it's a $1.4 trillion dollar industry, according to the USPS.