Upgrade charge on BTT

When I try to update BTT from the app, I get a message telling me that I need to pay for an upgrade after 2 years, fine, not fine, OK. It says that the upgrade will cost me approx. $6.5. But when I try to upgrade from the website, the charge is $8.50. Not a big deal, but I'd rather not pay the extra $2. What's up??