That interpretation doesn't make much sense to me. Unless there's a human actually validating each order individually right as you checkout, you could just have a note saying there won't be any shipping on saturday. Or even that orders only ship mon-fri. And nobody would find it surprising.

Other commenters' hypothesis (against doing business on shabbat) make a lot more sense, a checkout would in fact be "doing business" even if the business does that on its own it's still in your name and under your responsibility.