Wirize - Payment checkout for freelancers
Slack Surveys by Standuply - A simple, powerful bot for recurring surveys on Slack 📝
Google Payment API - Enable an easy-to-use checkout experience for customers
BlockSurvey - Privacy-focused surveys, polls & forms
Pay Me Or Else! - How to avoid and deal with clients who don’t pay (ebook)
Boolean - Send single question yes/no surveys