So many things I don't understand; maybe I'm slow.
You were asked over a year ago and then the invitations came about 5 months later which would equate to the invitations being sent over 7 months before the wedding?? That in and of itself seems absurd. I've seen "Save the Date" cards sent that early, but an invitation being sent that early seems off to me.
I don't understand how people think she's "using" you; what would she be using you for?
It is typical (at least in the US) for bridesmaid to pay for their own dress. For this reason, the considerate bride chooses reasonably priced dresses.
I have been in a number of weddings, and now I kindly decline and it's never been an issue. One of best friends knew I was tired of the expense of being in weddings, so she asked me to do a reading at her wedding and included me in all of the bridal party events (such as the rehearsal dinner.) Loved that!
I'm always a "more than one side to the story" girl, so I'd be very interested to hear the other side. Not because I think you're making things up but I always find the other perspective helps to see the full picture more objectively.