I have an away ST (actually had one since 2000 - I should be locked away in a nut house). When it started it was great for us as we live 15 miles from Cov, but its just more of a convenience now. I've not decided if i'm going to carry on next season yet.....
The first requirement is that you MUST have a home ST, you cannot sign up without one. When you complete your home ST form there is a box you can tick to sign up for the away ST.
You pay for each ticket at face value as it goes on sale and they post the tickets out to you. There is no cost up front but it does cost you a fee each time they process a transaction. I pay by debit card and its 45p per ticket. Credit card is more but not sure how much.
Once you sign up for it you cannot cancel it for the rest of the season, they will take payment and send you the ticket regardless of whether or not you can go to the game.
If you have insufficient funds to pay they will not process it and you will not be sent the ticket.
Hope this helps!