If something is broken: what you did, what happened, and whether it was the website or the Windows application. If you are deciding whether to buy: ask anything, including whether it reads a database you have.
Never send a connection string, a password or a backup. A table and column name is enough to work out almost anything.
Billing, receipts and cancellation are on the Billing page. What we hold about you is in the Privacy Policy — ask above for a copy of it, or to have it deleted.