I use Gumroad/Leanpub to sell my ebooks. As far as I know, they don't support the "choose to pay after" model. You can see the number of paid sales for the bun

I use Gumroad/Leanpub to sell my ebooks. As far as I know, they don't support the "choose to pay after" model. You can see the number of paid sales for the bundles under the "I want this!" button. When the price is 0, it shows the total of both paid/free users. I started selling ebooks about 5 years back. Where I live, my monthly living cost is just $150. While the first two years of sales were just about enough to cover my costs, the last three years have been much better - I can continue being self-employed :)