Girl, I feel you...I am so sorry to hear your situation. if I were you, this may sound harsh, but your very good friends who said they were coming, tell them straight up that you thought it wasn't "kosher" for them to tell you they were coming, so excited, and then break the news in an RSVP... and the whole sending a check via RSVP is a little tacky. But you know what? You and your fiance are the important ones, the ones who HAVE to be there. And I understand the whole thing too, one of my best friends is a florist and has over $40k in student loans (thought she wanted to be a teacher but did what made her happy) and she cried to me about how she is doing everything in her power to make it, which I love her and appreciate her telling me all this... but then on the other hand, certain nameless important guests who have lots of money, and who we never ask anything of, gave us lots of greif...
Bottom line... it's okay to feel bad, but know that some of them, I am sure, really did try to make it... the others with the lame excuses (I have 3 other vacations planned), don't let it worry you... At the end of the day, the two people who matter will be there, and will have a lovely wedding!