I strongly recommend adding a life-time option to the currently available Upgrade option, namely monthly or yearly revolving subscription.
I believe, this would invite more loyal customers to the app which would then attract even more customers.
Personally, I would be very willing to buy a life-time subscription while I am currently hesitant to upgrade using the revolving options.
This is very similar to this other request
It would be nice to see them merged.
Igor Santos commented
I feel the same. I seldom use Memento, so it doesn't make much sense to pay monthly or early for it. However, I do like the app and it gives me much value at the moments in life when I need it, so I would definitely pay a one-time fee for it.
I do understand this doesn't help as much with continuing development, but... it's still better than just staying with the free version since monthly is a greater compromise.
I guess it would work with a reduced, basic feature-set for lifetime payments (less than subscriptions but more than free version).