As another poster mentioned, SmartPost is a FDX service that uses the USPS for 100% of the end mile delivery. FDX picks up the trailers from customers and brings them back to a facility to process smartpost pkgs. They then sort them into trailers headed for the BMC (Bulk mailing ctr). However, they don't release trailers until they are 100% full, so it could sit for a while on the door. Also, these aren't expedited movements, so there could be delays from the carriers. Then the post office processes it at their BMC and gets it to the local DDU (Destination delivery Unit). aka the local post office. Then the mail carrier delivers the pkg.
Our Basic service is picked up by UPS and brought to a UPS building. Usually we hold the trailer for one day to purposely delay service, so it doesn't compete 100% with our grnd service. Although for some customers this isn't done. Then we put into our network with normal delivery commits. 75% of the pkgs get delivered by the manwoman in brown, and 25% gets delivered to the local DDU (post office) where the mail carrier delivers it the next day. Our Basic is a bit more expensive then FDX's smart post, but it's a lot better service also.
Also, with our Basic (weather it goes to the USPS or not) has no claims, shipper release.