OP here. I find it interesting that despite paying $80 / yr for past 5(?) yrs, I completely forgot I pay for Amazon Prime (thanks for the reminder!).

Does that mean it's easier to maintain renewals for SaaS offerings if the service includes high levels of premium/add-on/normal purchases?

Dunno, but I'm going to re-read Dan Ariely's Predictably Irrational :)