I know many of you Americans send registered 1st class international letters to the US (from what I see on the Forum.) So, for example, from Thailand a registered 1st class letter to be sent to the US is what, about 120 baht? From the US it costs about $22.70 or about 390 baht to do the same in reverse. I would think if the US leaves the International Postal Union treaty, since a majority of the cost of delivering is in the US, the Thai price is going to become somewhere nearer the 390 baht since the US is going to charge Thailand Postal the additional cost.
Good things seem to never last!