USPS shipping rate confusion

hi,

our site will process payments with paypal. we will also shit using paypal's shipping option as it seems to be the least amount or extra work..

my questions are.

1. how do i get rates for my customers from usps?
2. How do we set box sizes for a product.
we sell all products that are similar in size so if we sell 1 item it will fit into a box 8 x 8 x 12
if we sell 2 products then it will fit into a 16 x 8 x 12.

can open cart determine how many items will fit in our box and charge accordingly. or can we maybe do a
shipping on 1 is 7.00 and 3.50 each additional or would weight based shipping be more our solution.

thanks