What is this exactly? On one hand you're talking about anxiety and sleep, and in that aspect this looks like just a spin on guided meditation, unless you have studies that show that weak breathing muscles are an issue in this case. On the other hand you're talking about an incentive spirometer, which is a medical device. An app isn't a medical device, and I doubt you can give any guarantees it'll work. Besides that, if I needed treatment, I'd expect a licensed professional to give me exercises, and since the physical device is like 10$, I don't see how your 15$ subscription is competitive. Especially that breathing exercises can worsen some medical conditions.