Our Cat (Craig) left an excellent comment on the last post. I know that not everyone reads comments, but this one had valuable content and shouldn't be missed.
So here's what Cat shared (taking her info from the USPS):
U.S. First-Class Mail® International Letter maximum length is 11-1/2" long, 6-1/8" in tall ,or 1/4" thick.
I have a piece of cardboard that I've cut a 3/16"-wide slit in. If my card slides
through that (even if it's a tight fit), I know I am good for standard
postage. (GREAT TIP, CAT!!!)
Square envelopes and rigid envelopes (non - machinable) cost
just a bit more.
Standard U.S. postage right now is $1.15 for "normal size
and shape" envelopes; rigid or square costs $1.36.
Anything 1.1 - 2 ounces costs $1.98 to $2.19.
Once you get beyond that 1/4" thickness, regular-size envelopes (whether US or International) are classified as packages and the
price goes up quite a bit.
If anybody else has any information they can copy and paste into a comment from their country's postal office, we can create a database for easy reference.
Deborah, Gail & Jo