I wouldn't be over the top, as others have said, you may have mutual friends/not want to be as nasty back as she has been to you. You are in danger of losing the moral high ground.
I would just write:
'I am really disappointed not to be invited to your wedding after I saved the date. I just wouldn't feel right coming along to decorate and then disappearing, so I won't be helping out.'
Tidy.
I know some people don't think you should put you've been hurt by this, but clearly you have so I would be honest. What's to be gained by a formal or nasty email, she was a friend and if she has an ounce of gumption will see that she has hurt you even though she didn't meant to.