Some bakeries, like AZCouture, offer a comlimentary 4" or 6" cake on the couple's 1 year anniversary. The cake is not prepaid, because it is complimentary. And in some instances, the couple forgets about the offer after a year.
I use to offer a complimentary 6" round on a couple's 1 year anniversary. They had to contact me 2 weeks before they wanted to pick up the cake (everything is made to order, so I wanted time to make sure I had what I needed to make their cake - especially if I didn't have any other orders the week they wanted the cake). About 60% of the couples contacted me for their anniversary cake - the rest either forgot or maybe weren't married anymore.
I quit offering the anniversary cake because, frankly, I found it to be a pain in the butt when I had to make them. Plus, the freebie didn't seem to be the deciding factor for couples. Either they were going to book with me or not, the anniversary cake didn't factor into their final decision.
At a consultation I ask the couple if they plan on keeping the top tier. If they say "no", then the total servings is equal to their total guest count. If the couple says "yes", then the total number of servings is different than the total guest count. I explain that the number of servings is more than the guest count because "x" amount of servings is what the top tier would serve, and even though they don't plan on serving those slices, they still need to pay for them.