Building a JavaScript Tool to Calculate True Etsy Margins

When I first started selling my handcrafted leather goods on Etsy, I thought I had my pricing figured out. I calculated my materials, added a bit for my time, and slapped on a margin. But at the end of the month, my bank account didn't reflect the sales volume I was seeing. I was puzzled until I dug into my Etsy payment statement. The hidden costs—transaction fees, payment processing, and the dreaded offsite ads fee—were quietly eating into my profits. I was essentially subsidizing the platform's marketing budget out of my own pocket. I tried using a generic spreadsheet, but it was clunky, hard to update, and didn't account for edge cases like varying quantities or offsite ad opt-outs. So I decided to build something. The Problem The core problem with pricing on marketplaces like Etsy isn't just knowing your costs; it's understanding the compounding nature of platform fees. Most sellers just look at the 6.5% transaction fee and call it a day. But the reality is a multi