Can you add a final purchase feature? – Recolor: Coloring Book. Review

I like this app, but I don’t want the premium or whatever to be just for a month or year. It would be very, very convenient if there were a final purchase feature. But I wouldn’t want it to cost too much. Maybe it could be about 10-20$? This is just a suggestion, but I hope you hear it. I really want this feature.
Review by The Mug Life App Advisor on Recolor: Coloring Book..