[AlittleSalt]

I looked on USPS sites and didn't find what I was looking for. However, I found this at http://www.nerdylorrin.net/jerry/postages/

Small Packages / Parcels / Lumpy envelopes 1st class
These are padded envelopes over 3/4" thick, rigid photo mailers, medical samples, small boxes under 1 cubic foot. If it's not cubic but very rectangular, then girth + length have to be under 108".

1 oz $2.67
2 ounces $2.67
3 ounces $2.67
4 ounces $2.67 and add 18¢ each additional ounce, down from 19¢ last year
5 ounces $2.85
6 ounces $3.03
7 ounces $3.21
8 ounces $3.39
9 ounces $3.57
10 ounces $3.75
11 ounces $3.93
12 ounces $4.11
13 ounces $4.29
14,15, and 16 ounces were announced as acceptable on 7Jan2016,
but this is only for COMMERCIAL First-Class Package Service, not retail service.