I actually had a very good year with USPS. They were scanning things more frequently, up until the couple of weeks before Christmas, when the first scan was at the sort facililty, then at the destination facility. But only one lost package, and it showed up badly damaged, the same day the replacement one arrived anyway. The customer was sweet enough to ship it back and I was able to salvage 1/2 for another order.
If you are using envelopes, maybe they fall off a belt easier or something, and you can check the price of shipping in a small box. I think it's the same for under 1 lb. Maybe the box will be less likely to get lost.
I have also found the smaller post office near me to be more reliable. The ladies there have gotten to know me, and they always scan my items. The only things that have gotten lost or never scanned were in the larger post office near me.