Other ways to save costs:

Through folding: ounces flat in a 9x12 is $1.98; folded in a #10 it's $1.11. Square envelopes pay $0.49 extra.

Print package labels online: a 12 oz package coast to coast is $13.65 at the counter, $8.95 through USPS's free Click-N-Ship.

Keep boxes at or under 12x12x12. Since July, anything bigger is billed at volume ÷ 139 pounds if that exceeds its weight.

I've been building an (unofficial) searchable reference to USPS's rules and prices. The presort page is here: https://postalref.com/guides/presort