this is the correct way, fast, no transaction fees, no one takes a cut

And safe and easy. Cash in an envelope could be lost, and then it's gone. A check could be lost and would need to be canceled and re-written. And either way you have to take an extra step to get it in your bank account. (At least with a check, most banks have a phone deposit option. You'd have to physically take the cash to a bank branch or at best ATM.)