I don't see why anyone would put your decision down. It is your life. Which means, not ours, not your husbands, but your own! Yes, you do share your life with your husband, but your body is your own. I hate to say it, but if you are already thinking this would cause a divorce, then you two might have other issues besides this one. But, again, you have to do what you have to do.
Everyone here on earth has to do whatever it is inside of them that gives them comfort to live their life to the fullest. If your husband cannot understand that, then he is being selfish. I don't know him or you, and I don't mean to call him names, but just trying to give you some thoughts you might be able to use in your letter. I do think the letter is a good idea. It is how I have to talk to my husband too. If I tried to do it face to face, he would walk away and I would never get to finish.
Just make sure you tell him that you love him and need his support and don't bring up a dovirce or anything just yet. See where the letter takes you first. He might just surprise you. Also, attach to the letter, all the research you can get a hold of. Like cost in the US compared to Mexico. Show him that this surgery has been done in Mexico for over 15 years and the US only 5. Make sure you tell him that Dr. Ortiz is the one who taught the Americans how to do it. Just get as much info as you can to include wiht the letter so he has it all together and can read it all at one time.
Then, if all else fails, send him here and w'll take care of him, LOL!!!