RockAuto shipping weirdness

I know there are plenty of complaints and words of caution about the shipping charges with Rock Auto.  Having said that, for the most part I've found their charges to be relatively fair, prices good, and I like the variety of options that they usually have for a given part, from crap to factory or better (sometimes).  

Having said that, I do play around w/ the parts in my cart to try and minimize the shipping costs.  I try to do more maintenance than repair, so often I don't absolutely have to get a specific part, which affords some flexibility.  Anyway, yesterday I was starting to piece together some brake parts, and I started with what I wanted, front rotors and brake hoses.  Checked shipping, and they were all shipping from the same warehouse, but cheapest shipping cost was $21 on a $45 order.  I then added a pair of indicator lights for $0.79, and total shipping went down to $11.  Puzzled, I then added an oil filter and a set of injector seals, and shipping went down another dollar.  All still from the same warehouse.  So in the end, the total cost including shipping was like a dollar less with the other stuff than if I had just done the rotors and hoses. 

I have no idea what the algorithm is that comes up with this result, but I'm not complaining.   

That is interesting! I'll try it.

I have never experienced or at least noticed that, and my fridge is completely covered with rockauto magnets except for the bottom foot or so. 

Yep, their algorithm is whacky - I need to get stuff shipped down to Peru, so I'm constantly playing around with it, and I've seen this too, more stuff has occasionally yielded cheaper shipping.  I'll normally have half a dozen iterations before I place an order, to try and maximize the savings!