Unless you wanna send your stuff to the Phillipines.... then its a pretty good deal. I've been to his shop, I've seen his leather kit in his personal super coupe, and it looks good. Only place I know that sells replacement leather. I've bought stuff from Bill, had lengthy conversations with him. He is a nice guy. But like everyone running a business he also is out there to make money. I've bought things off him for the suspension of my Mark. No problems yet. But, I keep all my recipts in a lock box. He has layaway which is nice.
Some say his parts are substandard. Mostly his suspension parts.
However he has loads of name brand stuff in his office. And even sells some nice 1-off stuff you won't find many places.
Not telling to shop or not shop there... Just make an educated decision when buying. People are quick to jump on the "bash SCP bandwagon" around here.
Shop around. Compare prices. Talk to upholstery shops in your area with a good reputation.