ShortMax Subscription Charged $19.99 After Free Trial Cancellation

I downloaded the ShortMax Innovations mobile app and started a free trial through Apple subscriptions. I cancelled the subscription immediately when the free trial ended, and after cancellation I no longer even had a user ID or access to the service. Despite this, I was still charged once for $19.99 through Apple subscriptions. I have tried to find a way to contact ShortMax Innovations but I have not received any response and I do not know how to reach them directly, which makes the situation even more difficult. I clearly cancelled on time and I am not using the service, yet the payment was still taken. I request a full refund of the $19.99 that was charged after cancellation and I want my subscription to be permanently cancelled so that no further charges are taken from my account.