You shouldn't think of any tasting as a bunch of layer cakes. It makes a lot more sense to give people some neat chunks of just cake and some teaspoonfuls of the different icings and/or fillings that you would recommend. You put them neatly onto a white dinner plate and give them a real fork, that makes more of a difference. Let them mix and match.
And for your home based business, bake up a dozen mini cupcakes of each basic flavour when you are making the cake for any customer, and freeze them. Instant cake tasting stock.
So this particular customer wants your work for free. WHY do you think you should work for FREE??? Please--your state has a minimum wage law and you should be asking at least minimum wage for your time. And when people comment on your prices, you say "sorry, minimum wage applies".