Unfortunately this great app is not free. Instatext gives you a very short 14-days free trial that is limited on 20 000 characters, and then charges 10 EUR per month.
I am sure there is a free alternative somewhere because I don't have over hundred of dollars to throw away on a language application, even if it is very good.
I tested the app and it works perfect. I corrects all stuff right away, gives you suggestions and the whole process is very fast, almost instantaneous.
They only accept payments by credit card and PayPal, and all verification are automatic. I hate when I find a perfect app and it requires tones of cash to pay in order to use it.
Grammarly is not free either, but it at least gives you some basic functions to keep on using it. Maybe I will come up with the better option in the future.