Honestly, it can be a lot more of a hassle. In the US, if I'm selling a pickup/pedal/small item to someone else in the US, I can use an automated kiosk to ship the package and drop it off at whatever hour of the day I feel like without having to worry about lines at the post office.
If it's going international, I need to get and fill out a customs form, abide by the post office's conservative business hours, and wait in line to ship it. It's the difference of a 5 minute trip to the post office at my convenience to a 35-60 minute trip (depending on the time of day) that has to happen between 9am and 4pm, with a window from 12 to 2 where the lines will be 20+ people long because people can only ship on their lunch breaks.
For larger items (amp, guitar) I'd use FedEx, but they charge way too much to consider for international shipping. Then I have to worry about the size of the package abiding by USPS guidelines (which is different for every destination country), and repeat the process outlined above but while carrying that huge package.