You understand that sending 1 GB can take a while, and 20 - 40 Gb a very long while, so I doubt if it would be worthwhile.
If you want to try, you need to walk the client through using GHOST etc to do an exact image or send a batch file that they can run to do it in DOS, and image to an external USB or other drive, and then get the file to you somehow.Email will not work, so you would need to have a secure site that uploads via password are possible, at yor location.
In all, Fedx type delivery will be MUCH faster !!
I hope this helps !