Wirize - Payment checkout for freelancers
PeachPay - Convert your passion into income
Google Payment API - Enable an easy-to-use checkout experience for customers
ChimpCharge - Super simple invoicing for freelancers
Pay Me Or Else! - How to avoid and deal with clients who don’t pay (ebook)
Free-Invoice.co.za - Free Invoice Software | Free online tool that helps small businesses to generate free invoices and manage their contact list. It can be used as an alternative to expensive invoicing software and online invoicing applications