It probably also depends if you are only shipping within your own country or internationally. I use padded envelopes to send within the UK and sometimes to Europe if the pony is sturdy and I can pack it well, but not often do I do that to more distant destinations. And yeah, the obvious about delicate ponies and boxes you probably already realised xD.
I stockpile boxes from items received and recycle them to send items out. At least, I did until my dad started commandeering my box spares to send copies of his book to places that ordered them. Sigh. All my nice Japanese boxes are gone :(.
I can't help in terms of USPS pricing.
Yeah, buy bubble mailers in bulk! Last time I bought these: 9" x 14.5" bubble mailers (https://www.amazon.com/gp/product/B000GHZZV6/ref=oh_aui_search_detailpage?ie=UTF8&psc=1).
I can fit two adult G1 ponies and one baby G1 pony in each, or three G1 adults depending on their pose.