I've been building Invosmith , an invoicing tool for freelancers and small service businesses, on Next.js + Cloudflare Workers + D1. It's my first real product, and most of the interesting problems weren't the ones I expected. Here are six of them. 1. "Was my invoice opened?" — tracking without lying to the user The most-requested thing from freelancers is simple: did the client even look at it? The obvious answer is an email tracking pixel. I didn't rely on it. Apple Mail ...