It's always a good idea to go online to USPS to double check international postage. The price has changed over time, both up and down. While you're online with them, you can also purchase global international Forever stamps (which they had stopped issuing for a while, but they're available again - yay!), which are round and have a golden moon in the center. Pretty cool looking!
And, you can also stock up on sheets of the first class Forever stamps of the National Parks. They're almost too pretty to use, but I'll use 'em eventually.
Maybe part of the problem was you used two Forever stamps for first class mailing within the USA. Without an actual dollar amount on the stamps, it couldn't be applied towards the price of international mailing. And international is priced by the half ounce, not the full ounce that it is in the States.
However, good news it got returned to you, hopefully in a timely manner. I've had stuff take a very long time to get returned to me for one reason or another.