So using US domestic vendors I've had pretty good success with:
Buy BTC Monday Order Tuesday Order ships Wednesday Package arrives Friday or Saturday
However this week a pack that the vendor claims shipped Wednesday has yet to arrive (non FE btw). I'm slightly annoyed as the vendor has a reputation for fast shipping, the item is one that costs more on DNMs than IRL, and it's a product that really should be shipped fast if you know what I mean.
Is this common? Do I need to start getting orders in Monday for delivery by the weekend? Or is this more likely just a USPS fuckup or a vendor marking things shipped before they actually ship?
Yes. You are extremely lucky to have a high rate of success ordering Tuesday and having them all arrive by friday/saturday.
Vendors mark things shipped before they ship all the time partially due to opsec.