We use a few (we like to keep things complicated)!
Both UPS Mail Innovations and DHL Global Mail are a similar service.
They pick up the orders from our warehouses and do the first leg of the journey to their destinations.
UPS/DHL will then transfer the packages over to USPS who will digitally accept them and then complete the delivery.
This means that although you see UPS or DHL tracking, the package will be delivered to your mailbox with the rest of your post as normal.
Similarly for international delivery, UPS or DHL will ship to the country of destination and then packages are transferred over to the country's public postal service for delivery.