QuickReceipts - A mobile app that automatically tracks, organizes, and stores your digital receipts privately on your device.
Envoice - Send invoices and get paid on Paypal, Payoneer, Stripe and Square.
Receiptful - Receipts that increase CLTV. Exclusive (early access to PH)
Invoice Ninja - Invoice Ninja is a free, open-source solution for invoicing and billing customers.
Keep Your Friends Close - A free e-book about maximizing Customer Lifetime Value
Invoice Bus - deliver all your invoices, price quotes, follow-ups, and thanks notesautomatically