I also ship International First Class. I use a small ($15) scale to weigh the item and all the packaging with it, and use the USPS website to determine how much it will be. So far I haven't had an issue, but then again everything has been under 13 oz.
We also use a DYMO 4xl label printer to print our labels, which makes it very convenient. I save 21 HOURS a year with my label printer vs printing them on the inkjet, cutting them out, and taping them to the package, AND filling out the cn22 (customs) form and driving to the post office. They run around $250 last time I looked, but are completely worth the investment if you can afford it!