Adafruit and lots of companies use UPS because they sell enough stuff to get discounts on shipping costs. Also, a lot of companies use UPS not necessarily because they're cheap, but because they have a good electronic system which makes life easier for companies... for example, you can print a label with all the information and have a tracking number automatically (more or less) and when you schedule it, UPS will come and pick the packages.
If i were to sell some stuff, I honestly don't know if I'd use the national post here in Romania, simply because it's just too much hassle. Let me give you an example..
They don't have procedures for companies that sell small amounts of stuff
Let me give you an example, suppose I have 20-40 packages that I want to send in the country or European Union.
I'd have to go with 20-40 packages (or whatever amount), ask the clerk there to give me 40 forms and 40 copies and for each package, I have to fill the form BY HAND and the copy with my address and the deliver address and the contents of the package (even though for EU it's not required). No, I can't download the form and make a script to auto fill addresses because that's too Then, i have to wait in queue at best 30 minutes to get my turn because there's only one or two clerks and the national post system still uses slow dot matrix printers and slow computers, and they refuse to upgrade or hire more people because they want to privatize and sell the company cheap - but that's another story.
Once I get my turn, the clerk there has to type on the computer my address and delivery address for each package, even though I filled the forms already by hand and they weight each packag, because who knows, maybe I try to scam them and steal a few cents worth of stamps. That's 3-5 minutes for each package - I once waited in queue about 25 minutes after a guy with 8 small envelopes in his hand.
So imagine people waiting after you, and imagine how much the clerks enjoy doing this - it's practically impossible.
Then for packages going outside EU, I'd have to go across town to the customs office, which is only open with the public for 4 hours a day, from 10 to 14 am. These guys also have their stupid rules: you must go with the packages open so they can check that the contents matches with what I write on the forms, so forget about packing stuff and adding protective material and so on because they'll pull stuff out if they want to. Then there's another stupid rule that I think only in this country is present: the boxes must have no inscriptions on them whatsoever, no company logo, nothing. If you go with a recycled amazon box or something, they give you white paper and tape to cover the box in white paper.
So yeah... it's cheaper? yes.. is it really worth it? if you send more than 2-3 packages a week, no, it's not worth it.
and then have the employee there weigh each package and type in the computer my address and the destination address for their records, AFTER I have to fill a form and a copy of the form with the same information by hand. I can't download the form from anywhere, they don't allow you to give them typed forms (i asked), they have to be filled by hand just because they say so.
Very few companies here actually use the local national system to send packages, it's more convenient to use couriers even if they're a bit more expensive. The couriers have electronic systems similar to the ones the big UPS or DHL or similar companies use so you get a lot of value compared to the shitty national mail system