I rarely visit the post office nowadays, but, in my experience, sending via 'letter sized' envelopes is a reliable mode for getting orders to customers, and they typically register with PB tracking later in the evening (or early the following morning).
That isn't so much the case with large envelopes, aka 'flats.' The ones I send typically do get delivered (albeit sometimes after the estimated delivery window), but they often don't register with tracking till they reach the destination facilities, which means there are three or five days of 'pre-transit' status over which to fret.
5 flats, all the same size and weight, might go out one day, and all but one will register with tracking, then the next day, 3 will go out and none will register; Can't figure out what the deal is... a bit of consistency is too much to expect, one reckons.
Anyway, I'll keep using 'letter sized' envelopes, but once my supply of large envelopes is gone, unless something changes for the better, it'll be Ground Advantage from there on out.
As an aside, I make my own packaging with single-face corrugated cardboard. The result is *flat* but nothing that can be sent 1st class. Ironically, the final weight is less than it would've been if, instead, a paperboard mailer was used... thus, in many instances, the price difference between 1C and GA isn't overly objectionable.
None of that is of any real help concerning your situation; Just thought I'd contribute my 2 bits.
Regards.
