Customers are required to apply for a license which includes a pre-purchase agreement of a number of uses.
Packages are available in 100, 200, 500 or for higher numbers give sales and ring on 0161 777 2900, and I'm sure they'll do you a deal
CoSign is designed to replace the "in person" signing of a document.
To read more about digital signatures, click here, but remember that wikipedia can be edited by almost anyone.
The key is in the delivery of the key (sorry).
The protocol that you use to provide them with the key must be a method that you trust. Select a mechanism, that you can rely upon. For instance, if you have a valid mobile phone number you can message them the key, what's app, or other messenger service.
Use a portal that your can communicate with them on, or if all else fails, phone them, authenticate them, and read it to them.