Consider using a cookie for that purpose. I was looking for the calculator everywhere until I realized I had to enter my email first. That's a pretty big trust hurdle – no thanks.
I suggest you let people use the calculator freely, and when I enter e.g. "4 transatlantic flights per year" then you show me what that sin translates to specifically for one of your offset projects, e.g. "1000 briquettes" (with photos, of course). Personally speaking, that would make me pay up.
Oh, and the 20% fee is really problematic. I'd like to see a pay-what-I-want slider where I can adjust the percentage. The slider minimum would be set to min(0.2*x, $10) or some other reasonable amount, and you can calibrate that function to maximize donations.
Your footprint should be available on that page—you should be able to see your annual total, your breakdowns by category, and how that compares to global averages. The "Email me my footprint" button just sends you a link so you can save it. I could see how you may want to change the email you entered once you get there though. We should add that!
I just ran through the page after seeing this comment and was also confused. To me the phrasing and styling of the "Email me my footprint" button makes it feel like the end of the process and that I need to click it. It didn't immediately occur to me that I should keep scrolling down. In my case I happened to scroll so that the "Step 3: Offset" was just out of view which made it feel even more like an "OK you're done!" button, but I think even if I saw there was more I would've wanted to click the button expecting that to be the way to see my results. Maybe the two of us are outliers but it might be worth it to explore some small design tweaks here. Love the concept!
