Well crap, I just adviced to make sure they weren't stupid and stand in the wrong order or forget to pack undies. This is WAY beyond stupidity.
Here's my problem with people in general. Not just annoying wedding guests and stupid bridal parties. People tend to say "ya sure! I'll be there" but not really stick by what they said. Flakiness is all to common and it's just downright ridiculous. There is no excuse for it. If you saying you are going to do something, then F*ing do it!
Unfortunately, the time we get to learn this lesson the hardest is during our weddings. The way people really are becomes apparent really quick!
Now, there is a huge difference of telling you first, sure they would like to go and then changing their song a year ahead of time. YOU LEAVE IN A MONTH! I know I wouldn't have $4000 sitting around to cover this expense. I hope you make it well known how much it's costing you because that is ridiculous. At this point, they need to suck it up and whip out their F*ing CC.
Sorry- this crap makes me irate. I deal with flakiness all the time from a couple of my friends and at any level, it's not ok. Do what you say you are going to do! Period!