Here’s the current prices in the United States:
Domestic Letter (1 oz.) 58 cents
Domestic Letter (additional ounce) 20 cents
Domestic postcard 40 cents
International letter (1 oz.) (includes postcards) $1.30
As of July 10th 2022, these rates will increase as follows:
Letter (1 oz.) 60 cents
Letter (additional ounce) 24 cents
Domestic postcard 44 cents
International letter (1 oz.) (includes postcards) $1.40
The international rate is the same regardless of whether you send a letter in an envelope or a postcard as long as it is under one ounce and meets certain dimensional standards. The international rate varies after the first ounce; the world is divided into zones, and the rate for additional postage depends which zone the country you are sending to falls in. More info can be found here:
International first class rates
International country price groups
As you can see, it is somewhat complex to figure out. I built a spreadsheet for when I send packets of cards over an ounce.
The Unites States Postal Service (USPS) sells “forever” stamps, which sell at the current rate of postage, but remain valid regardless of what postal rates may increase to. Thus, it is always a good idea to stick up on stamps of this sort.
Hope this provides what you were looking for.