[ SYS-003 ] // FINTECH SAAS // CALCULATIONS

Microfinance Loan Calculator Platform

Built a standalone SaaS calculator for loan officers and microfinance institutions across Kenya. Supports EMI, reverse, forward, buyoff and top-up calculations with fully editable deduction structures, tiered subscription access, and integrated M-Pesa payments.

Astro Django REST JWT Auth Daraja API PostgreSQL

// TARGET_CONTEXT B2B Subscription Tool for Loan Officers & SACCOs

// ARCH_TYPE Decoupled Frontend (Astro) / REST API Backend (Django)

// DATA_LAYER PostgreSQL-backed subscription and usage tracking via shared ERP infrastructure.

// SECURITY_PROTOCOLS JWT-authenticated API access with tiered rate-limiting enforced server-side per subscription plan.

01 / The Pivot

A calculator engine originally built for an internal ERP system was extracted into a standalone product. The insight was simple: the deduction and EMI logic loan officers needed daily had value independent of the full ERP — turning an internal tool into a sellable subscription product.

02 / Tiered Access Control

Free-tier users get limited daily calculations enforced server-side, not just client-side, preventing trivial bypass via local storage manipulation. Pro subscribers unlock unlimited calculations, reverse computation, buyoff structuring and top-up logic, gated behind JWT-verified subscription status checked on every request.

03 / Local Payment Rails

Integrated Safaricom's Daraja API for M-Pesa STK Push, allowing subscribers to pay directly from their phone with PIN confirmation, alongside PayPal for international billing. Subscription state updates automatically via asynchronous payment callbacks.

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