For outgoing packages, I always use USPS Priority Flat Rate Boxes or Priority Flat Rate Padded Envelopes, except for small bubble envelopes which are mailed USPS First Class (with tracking).
I've shipped several dozen packages CONUS over the years with zero issues, outgoing or incoming (easily 50+ inbound). Same for CAN and AUS, both ways -- no problems so far.
Europe has been a different experience, however. Two SFRBs to France disappeared (I ate the cost on those two), one to Finland disappeared (insurance covered it), and one package from Sweden took 6 months to finally arrive (seller had already shipped replacements after two months, which arrived in two weeks).
Four strikes, out of 10 total packages to/from Europe. That's why I no longer ship there, unless the buyer is willing to absorb all losses.