Have searched through every document in your module and in none of them does the string 'intvar' appear. Lots of 'intvals, but no 'intvars'. Cleared all caches. Still getting the fatal error. Are there more compatible modules I could be using it with? (July 3rd, 2008 release 5.x-1.9)
Just downloaded and installed the 5.x-1.x-dev and it doesn't have the same problems. What's the difference in functionality? What's stable in the non-dev release that isn't in the dev?
It's great, btw. I love being able to pay with userpoints exclusively. Have a great weekend!