Well if any of you have had any shipping experience, then you know with a larger corperation they don't ship out each package individually!
I used to work for Jeg's high peformance in Columbus. Our contract with UPS was for semi trailer loads. The packages would sit on our dock until the semi was full...and I mean not an ounce of space left full. Only then would UPS send a cabover to come pick up the trailer and take it to their sorting facility. We could not list the products in our system as shipped until they left our facility. Often times when someone called me on the tech support line I would go into the warehouse to double check and see if we actually had something on the racks because I knew even though it said we had 2 or 3 in the computer, they might be on the dock waiting for clearance to ship.
Chances are things are the same at PSI. The only problem may be that they aren't as large as CSUSA, so it takes them a bit longer to fill the semi trailer, and hence slows down the shipping charges.
That's just the way any corperation works that ships in large quanitities. I'm sure they could ship quicker, but it would cost you more as has been noted previously. The only way you can get away with the small shipping costs that PSI charges would be to have a value contract like I stated above.