USPS is the way to go as long as the local Postal Service in the receiving country is reliable.

Example: Kuwait=No I still have a Box MIA from back in 1996.
India=Yes slow but it will get there eventually. I send to my mother-in-law this way no issues yet.

All USPS mail rides on DHL or even sometimes Fed-Ex / UPS planes to get there anyways.

All I would suggest is make sure you get Insurance and some form of tracking and Signature Receipt. Which are extra but very minimal.

Far as companies only using UPS and FedEx that's because they are just plain too lazy!!! USPS does have a pick-up service but you have to actually PickUp the phone an schedule a Pick-Up. They don't just come around pick it up every day like UPS/FedEx does with businesses. If the business has their Mail Delivered directly to their shop, they can just as easy ask the delivering person about a pick-up.

I work overseas a lot and it is extremely expensive to get things sent out in the places I work via DHL or Fed ex (UPS is too scared to come here LOL). I won't order form anyone that doesn't ship via USPS.

Used to be DHL was the way to go for Int'l shipping. They went places no one else would go and the prices were cheaper. But they closed their US Operations (even though they were an American Company). They will ship into the US. But I don't think they do any shipping out any more.