IF the buyer was made aware of 2 different prices at the time of purchase, one for the Wedding Cake, and one for the Groom's cake, then I would suggest only refunding the amount for the damaged product, plus any delivery fees for both cakes.
IF the final contract between you and the bride did NOT show an exact price for each cake, then I'd refund the entire amount for both cakes. Now is NOT the time to be providing a new invoice which breaks down the pricing for each cake.
Here's the new wording suggestion for the new email and letter IF the contract specified an amount for EACH cake:
"I take pride in my bakery and the wedding cakes I have delivered to many brides in the past, but, unfortunately, cake is fragile. The Wedding Cake was damaged while the Groom's Cake was not. I am sorry the Wedding Cake for your special day was accidentally damaged, and I am enclosing a check with 100% of the payment made by you for your Wedding Cake. I am also refunding the entire amount paid for delivery. I wish you and your husband many years of happiness."
I would attach a copy of the original contract which states the specific price for EACH cake.