Also, to address the most recent comment, my method involves no "software" of any kind. Also, stamps.com is a website you have to be a member of, and there are monthly fees. My shipping method involves paying for the cost of postage, plus a fee of a quarter (yes a quarter, literally 25 cents), printing your postage, applying it, and shipping. You can do it any number of times you want, the cost always stays the same.
Stamps.com is easy. Allows for quick First Class international shipping. ( And any other USPS shipping you desire. )
There is no "25cent fee" per package, so if you are shipping over 60 packages, you will cover the Stamps.com monlthy fee, and end up saving money on every package after the OPs guide he is selling. Stamps.com also gives you a free scale, and free postage when signing up. ( which takes all of 3 minutes ) .
If you end up shipping in volume through Stamps.com, they will waive the monthly fee all together.
And if you ship in high-volume through Stamps/USPS, they will also allow you to do First Class domestic mailing, up to 1lb for a first class rate of $4.05 .