This might seem silly at first, but read and bookmark/favorite. It will be a real time-saver.

http://www.antiquegamblingchips.com/USPostageNewRates.htm

The chart below (and it's in the above linked web page) saves you time. You need only keep coils of 45-cent and 20-cent stamps, and the chart tells you the number of stamps to put on your parcel mail pieces. (The new first class rate is 45-cents. By following the chart, you are usually putting on a few pennies extra -- because you are only using 45- and 20-cent stamps.)
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Below is the post office chart for the three types of first class mail:

Robert