Fairbill is an iPhone app for writing invoices, saving clients, and keeping drafts. There is no Fairbill account and no Fairbill server.
Subscriptions
Fairbill Premium is 1.49 US dollars a week, 2.99 a month, or 12.99 a year, unless the App Store shows a different local price. Apple bills the subscription. Cancel it in Settings → Apple Account → Subscriptions. Deleting your profile in Fairbill does not cancel it.
Delete your data
Open Fairbill, go to Profile, and choose Delete profile and data. That removes your profile, company, clients, invoices, photos, signature, PIN, reminders, and the iCloud copies Fairbill made.
Contact
Mark Kelly, Salt Pepper. For a question about Fairbill, reply from the support address on the App Store product page, or write through the developer contact shown there.