Mobile-first SaaS teams—especially the roughly 30 million mobile+web publishers—are repeatedly trapped by platform-specific in-app purchase (IAP) flows that make it hard to keep subscription state consistent across stores and web billing, causing revenue leakage, inflated support costs, and churn. When Apple or Google tighten enforcement or when client-side receipt validation breaks, engineering teams end up doing manual reconciliation, issuing credits, and rebuilding brittle ad-hoc tooling. You could build a server-side subscription sync service: a hosted reconciliation engine that ingests App Store and Play Store notifications, normalizes receipts and entitlements, maps them to web billing systems, provides audit logs and dispute tooling, and ships drop-in SDKs and stack-specific starter kits for common backend/frontend stacks. Priced toward the $167 average annual spend used to size this market, the opportunity is about $5.0B (30M publishers × $167), and the combination of subscription-first monetization and stricter platform enforcement creates immediate buying intent; market signals are strong (market score 95/100) and revenue potential is high (88/100). You can stand out by focusing on engineering-grade reliability (idempotent sync, durable webhooks), compliance documentation, and low-code developer experience—plus ML-based anomaly detection and a transparent, consumption-oriented pricing model—while acknowledging real challenges: maintaining fragile store connectors, adapting to evolving platform policies, and earning the trust required for billing-critical infrastructure. Competition is medium; many incumbents are web-first or lack robust server-side store sync, so a narrow, high-quality launch to SMBs and mid-market publishers with strong case studies is the pragmatic path to validate demand before scaling.