You are mistaken about them lying to you. They do know the legal requirements and you do not.
What you ask is not ab\available in a foreign country, even just over the border. You must have the postage stamps of that foreign country to be able to do a self addressed return. After all. the mail is not in the USA.
Global Forever stamps allow you to send a 1 oz letter from the US to any other country in the world (can be up to 2 oz for Canada). However, you cannot use them to send mail from any other country back to the US. http://about.usps.com/news/national-rele...
The Global Forever stamps are good for mailing letters from the US to foreign countries. The receiver of the letter will have to use the postage of his/her country to return the self-addressed envelope to you.
No you cannot do this as US postal stamps are not valid outside the US.
No they have no value outside the USA
