Bridesmaids out of state - letter

How do you tell your long distant, best friends you want them to be your bridesmaid in a LETTER? I've already created super cute proposal boxes, but I plan on formally asking them in person. But, because they're long distance and have kids, families, jobs etc... I also dont want to suprise them an make it a public announcement if they're not able to. The distance also has made keeping up with the relationships difficult but whenever I text them, its like were right where we left off.