Improve People’s Pension Integration – Merge Files & Fix AVC Mapping

Allow seamless People’s Pension submission when both Tax Relief and Salary Sacrifice schemes are active. Currently, two files are generated and must be manually merged since People's Partnership won't accept 2 files for one period.

Also, fix AVC mapping: if an employee has salary sacrifice AVC, it is incorrectly listed under employer contributions. It should auto-adjust to employee contribution in the Excel export.