Upgrading to the lifetime license is available, but the shop isn’t smart enough to do the math. Open a support ticket assigned to me with the serial number you’d like to upgrade (don’t include the license key, or include other sensitive data, unless you mark the ticket private), and I’ll add up the cost, and get back to you with the amount you can send to us via PayPal (or I can send a PayPal invoice for the amount, which you can pay by credit card, I think).
I’ll figure out how to handle it automatically eventually, but it’ll take some code to do the math and such.
I’ll contact you via email, Pete, with details of your particular case.