> We do this by monitoring their purchases for tax deductible expenses -- like phone expenses, insurance payments, and home office expenses -- using a team of accountants aided by fancy computer stuff.

Are you storing or otherwise processing the transaction records for any other purposes or data mining? Alternatively, do your terms allow that in the future?

Knowing every purchase a person makes is going to be a tantalizing data set.

> We’re currently piloting the service with +100 paying users. We charge $10 per month because it’s simple, but eventually want to move to a model where we charge a small percent of the tax write offs we find for you. Aligned incentives :)

Maybe not totally aligned though as it’d be in your interests to be as aggressive as possible with write off claims while the customer would presumably be on the hook for any rejections or audits.