The thing is, you are not going to get her to see how rude she is being. So, there is no point in trying to educate her. I would say,
"Dear Friend,
We have known each other for a very long time, and I feel I can be blunt with you.
I believe that asking me to decorate your venue is a big favour - the type you would only ask of a very close friend. I also believe that if we were such close friends, you would have invited me to your wedding, particularly after sending a "save the date" card.
You, on the other hand, believe that you are doing me a favour by asking me to do it.
We will not agree on this.
Please understand, however, that I do not wish to decorate your venue for you.
I wish you all the best for your day."
Tidy