HI Richard
It all looks great.
As you build it you need to make sure that everything will work with SQL Server.
This may be a good time to start introducing running the back end on a server like maybe My SQL or better yet Microsoft Azure. So that the POS can run from multiple locations.
So for the Signature Capture you can do that in Access. I will send you an email with an Access samples also some other access files that may help that you can use anyway you need.

Thanks
Ray White
Fog Line Software LLC