Add a payment-system (paypal yearly subscription) into an existing add-on for google spreadsheet

We have made an add-on for google spreadsheets but want to make this payable. That means that people can run the script for a certain time, but then should have the option to pay for a monthly subscription via paypal. The job is to send us a script we can paste into the existing script (js). Another possibility is that we go for a one time fee. If you want, you MUST communicate via skype. We ONLY look for people who have done such a job and have EXPERIENCE with google spread-sheet add-ons AND paypal API. You can also propose another payment environment.