Update application fee structure across documentation and codebase

Commit notes

- Changed the application fee from 30% to 15% in various documents including IMPLEMENTATION_SUMMARY.md, PRD.md, PURCHASE_FLOW_TESTING.md, and MARKETPLACE_FEATURES.md to reflect the new fee structure. - Updated the checkout handler to calculate the application fee at 15% and adjusted related tests to verify the new fee amounts. - Enhanced onboarding information to clarify the updated application fee for users.