USPS has screwed up plenty of our orders, but they are still the best way to go with large numbers of small packages & envelope type stuff.
We ship 3K to 4K packages & envelope's a year. Small flat rate for over 13oz is the best. Most of my padded envelope's are around 5oz, that cost is 2.65.
Don't think we could stay in business shipping small packages thru UPS.