Single stacked pallets or double stacked pallets?

We just received the following email from our rep at CustomCD (they handle the shipping of our CDs and barcode scanners):

I have a favor to ask – the last batch of scanners that was sent to us came on a double stacked pallet. Our receiving dock isn’t able to accept these, so they asked that the next shipment we get if it could only be single stacked pallets.

Single stacked? Double stacked?
I don’t even know what that means πŸ™‚

The joys of selling hardware, I guess. Sometimes, I long back to the days when we were just selling software, “shipped” by email.