Here is US Post Office Notice 123, good to bookmark it:
https://pe.usps.com/text/dmm300/Notice123.htm
It shows all the domestic and international rates, the retail rates (if you trudge to the post office) and commerccial rates (if you print your postage labels at home).

Currently it shows the current 2017 rates, which change in a week or so. But no sweat: when it changes, the Post Office also updates Notice 123 (check the date to be sure it is 2018). So you can keep using the same link.

Robert