Thanks!

Now I have to know the weight prices. How much stamps I need if I have envelope which weight is 30-50 grams?

I don't know how much something costs based on weight. I can tell you that you can use as many forever stamps as you want on a package. You can also combine internationals stamps with forever stamps. As long as you end up paying the correct amount of postage with either forever stamps or international stamps our post office doesn't care

Example: I have two envelopes; one weight 25grams and the other 50grams. I would want pay return postage with stamps. I live in Finland and would want send to USA. So, do I need one international 1,05$ stamp for 25gram envelope and two for 50grams envelope? Can I use any forever stamps or only international stamps? Can I pay the postage for 50 grams envelope with one international stamp and one forever stamps or maybe with 3 forever stamps? Is the max weight of one international stamp 28grams? Then it would be 56grams for two 1,05$ stamps?