Q: noticed that sockfancy.com is using shopify? is there a way to set things up like they do: user checks off the options they want and then they go directly to a single-page checkout for their subscription?
A: A subscription is a recurring order. For recurring orders they are using Recurring Orders & Subscription Box App by Bold, and Recurring Orders Enterprise, which is only available to Shopify Plus merchants.
Q: how about the single page checkout?
A: The single page checkout is also a feature of Shopify Plus plan, which starts at $2000 per month. They are also using cashier by Bold.
Q: It looks like those Bold plugins add commission costs of about 3% or more per sale?
A: I don't know the details of that, but again they are available only for Shopify Plus.
Yes it's true that you can set up a generic site cheaply, but for a site that actually works well and has single page checkout, etc. you get into some very big bucks, and also some very large sales commissions.