Subcontractor for an agency which develops this site.
I was one of two main developers for the software-architecture and development of a job portal. Users could either submit jobs, or earn points for work performed. The points would accumulated in a jackpot and monetary rewards would be dispersed at the end of each month.
E-commerce was done with Drupal Commerce.
I implemented the business logic of the client dealing with the calculation of the monthly jackpot payout, including transaction history. For this, the logic of a very complicated Excel spreadsheet had to be coded up, including a test suite (test driven programming) to verify that the system ran correctly.