Follow this Link to look into most detailed record of shipping costs. Rates are calculated inside the shopping cart before checkout.
( see below link... )
If you want to estimate " how much is the postage ? " Put the items in your shopping cart and proceed to 2nd page of the checkout.
During some sales, coupons are available , also the option to use coupons is on page 2 of the checkout system. There is a box to put the coupon code in.
Thank you.
https://pe.usps.com/text/dmm300/Notice123.htm#_c191