Know if clients opened your invoices — free email open + link tracking

Many small businesses and independent contractors—roughly 200 million globally—routinely lack visibility into whether clients actually opened invoices, which lengthens days sales outstanding and creates payment friction; with incumbents averaging about $90 ARR per business, that market represents roughly an $18.0B opportunity. The pain is acute for freelancers and micro‑firms that can’t justify complex AR platforms but need a simple “did they see it?” signal tied directly to payment status. A pragmatic product would be a freemium overlay that adds privacy‑aware email open and invoice link tracking into existing invoicing flows, with server‑side fallbacks and explicit consent options, plus tight webhook integrations to Stripe/PayPal so “view” signals can be correlated with payment events and automated reminders or attribution hooks. This is timely: the remote/gig economy is expanding demand for lightweight invoicing, payment stacks are increasingly API‑first (making reliable webhook correlation possible), and emerging privacy patterns favor server‑side, consented approaches over fragile third‑party pixels. The opportunity scores look strong (Market Score 92/100, Revenue Potential 88/100) and the product’s strengths are low friction, clear ROI for SMBs, and easy monetization via premium analytics and automation. Main challenges are medium competitive pressure from invoicing incumbents and email tracking tools, deliverability and client blocking by mail providers, and regulatory/consent complexity; success will hinge on flawless integration, transparent privacy design, and measurable impact on collections. If you can execute on deliverability and build native hooks into payment providers, this is worth pursuing as a focused, high‑leverage add‑on rather than trying to compete as a full invoicing replacement.