If they miss their 2-day shipping guarantee then you call CS, complain, and get a free month of Prime added. Ad infinitum. A way to increase your chances of missed 2-day shipping is to spread out your item orders so it gives them more opportunities to screw up. This also increases their costs of S&H across the entire logistics chain and maximizes the usage of your Prime membership. Plus it feels like every day is Christmas when a new random Amazon item is coming in.
Another key is to order stuff that weighs a lot (40lbs+). Last time I did FBA the UPS rate for Amazon shipping was ~50 cents / lb . So if you're getting a 40lb item (especially if it has weird dimensions) they're likely not paying much lower than $15-20 to get that to your door in 2 days.