Seeing as you registered on-line, use http://sales.oystercard.com/
and top-up there.

That wouldn't help as the top-up is loaded onto the card when you use the
gates or validators at a nominated tube station and the OP doesn't use the
tube. So the Ticket Stop is the only useful solution.

I have a similar problem in that although I do use the tube I don't use my
Oystercard with it so I would not be able to collect Pre-Pay or period
tickets bought online. I can't touch in without being charged for a journey
I am not making. I do have the option of using the ticket machines though.