Actually, with DHL in Germany, that’s exactly what many people do. The cheapest version is paying by wire and getting the code to put on the package via their website, and dropping the parcel at a package hub, the second cheapest version s paying by wire and getting the code to put on the package via their website, then dropping off at a DHL store, the third cheapest version is getting it stamped and dropping it off at a DHL store, and the most expensive option (about 20$ more per parcel!) is getting it picked up by the delivery guy.

Which is why DHL can offer prices such as 4€ for a cross-Europe 1kg 30x30x60 package.