The letter is very well written. I would liven up the headline a little. For example:
A Fun, New Way To Make Your Bridal Shower Party… Unforgettable!
Also, instead of saying,
"I'll even give you $20! Say,
I'll even pay you $20! In my tests, I found the word "pay" to be a more effective semantic.
You should also
bold or
highlight your guarantee.
Hope this helps!
Dale King