While McMaster shipping certainly isn't free, it's always overnight, and often less than 100% of cost, so if Amazon is actually 2x price, McMaster still makes sense.
Side note: McMaster has, hands down, the best/most useable site I've ever used. The flow always gets me quickly to exactly what I want, even if I don't know what that is going in.
I know this is the third "me too comment" for this, but you cannot overstate how awesome McMaster Carr is for this sort of thing - they have everything. Massive amounts of information on the site, super fast shipping, reasonable prices. If you build stuff in your spare time and haven't tried McMaster-Carr, you're missing out.
Side note: McMaster has, hands down, the best/most useable site I've ever used. The flow always gets me quickly to exactly what I want, even if I don't know what that is going in.