I think you mean, for showing a Box before finish the order that can show a "Terms and Conditions", and have to "Accept" the "Terms" before be able to finalize, is that correct?
If so, blesta alredy have that
In Blesta Admin, on Packages->Order Forms you have an option wen editing or adding a new order form called "Require Agreement to Terms of Service" and you can put an URL of your Terms os Service
After that, it will be show to the client the link for the Termos of Service, and he as to Check the box to agree and be able to finish the order.
Sorry didn see your title lol
Maybe a good thing like cpanel uses wen ordering new licencs is this: http://www.onlinesignature.in/draw-a-signature-online.html