I just sent some cards to Finland (FIN_ArtoD). One global stamp ($1.15) because it was under an ounce and one 21¢ stamp for the non-machinable surcharge (rigid toploaders). You can send as a letter up to 1/4" thickness with a 21¢ stamp for each additional ounce. Over a 1/4" and up to 3/4" is large envelope. I've sent a large envelope to Canada with four 3/4" stacks of cards spaced apart so the envelope would bend. It cost me $4.79. I've even split shipments into two mailers because it was cheaper than all in one mailer. Anything in a bubble mailer is automatically a package and the cost skyrockets.
I've sent cards to Canada, Finland, Austria, Australia, New Zealand, France, Spain, Germany, England, Czech Republic, Hungary, Indonesia. No problems, no outrageous costs.
I use the USPS Price Calculator https://postcalc.usps.com/. What will drive the price up is claiming value and using "Package". Some countries, any value over zero drives up to cost. Others have different maximums before the cost goes up.
Someday my cards may double in value and then be worth half of what I paid for them.