It's not that complicated, but reading the website, it can be confusing.

First-Class Mail Rates
First ounce $0.39
Each additional ounce $0.24

Nonmachinable Surcharge:
An additional $0.13 is required for items weighing 1 ounce or less with any of the following criteria:

A. Square letters.
B. The height exceeds 6-1/8 inches, or length exceeds 11-1/2 inches, or thickness exceeds 1/4 inch.
C. The length divided by height is less than 1.3 or more than 2.5 (length is the dimension parallel to the address).
D. It has clasps, strings, buttons, or similar closure devices.
E. It is too rigid or contains items such as pens that cause the thickness of the mailpiece to be uneven.
F. It has an address parallel to the shorter dimension of the mailpiece.

- Translation -

1 oz or less bubble mailer - 39 + 13 surcharge = 52c
(no surcharge above 1 oz)
2oz first class 39 + 24 for 2nd oz = 63c
3 0.87
4 1.11
5 1.35
6 1.59
7 1.83
8 2.07
9 2.31
10 2.55
11 2.79
12 3.03
13 3.27
14 3.51
15 3.75
16 3.99

Over 16 ounces use Priority Mail, rate varies by destination zone. Starts at $4.05 for 1 lb.

(old system it was over 13 oz, and their website still says that, but someone forgot to do the math)

Flat Rate envelope $4.05 regardless of weight or zone destination, sent in a USPS flat rate envelope free at your local post office.

Flat rate box $8.10 regardless of weight or zone if shipped in a USPS Priority Mail flat rate box, also available free at the post office.

Hope this helps?