Why on earth would you want to send a hard copy by snail mail? Email is instant and you have notice of delivery and proof of having sent it. It doesn't cost you postage, you don't have to waste paper and you don't have to schlep out and find a post box.
In short, don't ever use snail mail unless there is absolutely no other option.
That NtK does not even state a cause of action on it. It's a joke. It is an invoice that says, you owe us money without giving a cause.
When they issue the claim, come back and we will give you the instructions on how to acknowledge it and a defence to sue. If you follow the advice, you will not be paying a penny to anyone for this. I can say with greater than 99% certainty that this will either be struck out or discontinued.