Firing a bridesmaid?

So one of my friend that assumed she would be a bridesmaid, is extremely flaky. Since I never asked her to be in the wedding, and she's been so flaky, I don't really see why I can't just tell her I'm concerned about her behavior and I need people that are going to be there when I need them to be there.

I know she works, but she blows me off for stupid reasons.

Last month, she was supposed to go with me to Sacramento to look at dresses, she didn't show up. Luckily, one of my other friends went with me.
Also last month, she was supposed to go with me again to Sacramento to look at dresses, she didn't show up or call or anything.

She did go with me on Tuesday to look at dresses (but it wasn't planned, we were just walking by someplace and decided to go in), but I think that was just because she needed a place to do her laundry.

Then yesterday, she called me an hour before our dress appointment at another shop, that I had to book a week in advance, to cancel. So I ended up taking Aaron's grandma with me at the last minute

Tomorrow, I had made an appointment at another shop in Sacramento, and she was supposed to go with me. I made the appointment for 11:00. There was one available at 4:00 too, but she said that she had somewhere to be at 8:00, and I didn't want to be rushed, and its an hour there and back, so I took the 11:00 appointment. So when I was talking to her today, she was like I'll be at your house at 2:30. So I said why? Our appointment is at 11. So now she doesn't want to go, because she needs to sleep in on her only day off. So then she's like well cancel you appointment (shop is closed, already btw) and we'll go someplace in town at 3. No. I am not finding anything in town.

So I'm pissed. Now I have an appointment tomorrow in sacramento, that I'm going to have to cancel only an hour early - the shop opens at 10... unless I can find someone to go with me. Which I dont think I will. Its really short notice for an out of town trip for anyone down here that would go.

I'm just really pissed off.

So you don't want to go to an appointment that I only made because you said you wanted to go - first she said she wanted to go to San Francisco (farther away than Sacramento)!

I'm kind of in a spot, because she is really one of the only ones that seems to want to go (even if she never does), but at the same time - she never does end up going for some reason or another.