Amanda Bradford didn’t monetize The League for over a year and a half, but she still needed to show investors that users would pay. So she added a button: “Upgrade to Heavy Hitter.” Users who tapped it got premium features and were told their card would be charged. But the billing system hadn’t even been built yet. 15% hit the button, and that number went straight into the pitch deck. Lots more tips from The League’s explosive growth in a new TWiST Tactical & Practical…