The monthly was priced so the annual provided a 17% discount. At first, the split was 1/3 annual and 2/3 monthly, but this migrated over the past year to the current 50-50 (we expect this to revert back to the 1/3-2/3 split once the schools fire back up).
In the meantime, we added two other offerings, one of which was a set of virtual goods that could be bought separately or as a bundle, and the other were add-ons that augment the basic desktop app. These are one time purchases.
The combination of offering both subscription based services and one off purchases has worked well, with each model generating 1/3 of our revenue (the other 1/3 comes from a mobile app).
We often see users tip their toes in to the water by buying one of the lower cost "one off" items, and then come back later to buy more and sign up for the subscription service. Having a low cost item for sale seems to provide a mechanism for the buyer to check out the process before committing to a larger and/or repeating purchase.