I don't mean to be a hard-ass about it, but an invitation is just that...an invitation. It's not a subpoena. If you don't want to go, don't go. If you feel guilty about it, then those are YOUR emotions, not hers. I cannot complain when I feel obligated to go to something I don't want to go to. I'm not a 6 year old being dragged to a bookstudy or meeting anymore. I'm a thirty-something adult that has a life, which does not include spending loads of money on someone I don't know. If someone else has an issue with that, then it's their issue - not mine. And if they take issue with it and/or are rude about it, then perhaps they aren't as good of friends as I thought they were.
If it's too scary to think about it that way, there are all kinds of nicey white lies you can give to not go.
"Oh, I just got your invitation to Suzy's 3rd birthday party, but...
...my executive VP is in town that week and our staff dinner is that night." Or...
...I have a wedding to attend that day." Or...
...I have a huge project I'm working on at work and that night is the deadline. I know I'll be working late." Or...
...I have family in town that weekend."
You don't even have to say the lie if you just want to say you've "got plans". Those plans may be to sit on your butt and watch As the World Turns, but they're plans.
"Oh, I just got your invitation to Suzy's 3rd birthday party, but I've already got plans that afternoon."
I just don't understand it when people complain about getting invitations to something they don't want to attend. Why be controlled by family, friends, or office workers. Give your regrets and don't go. It just seems so simple to me.