Right. Think of it this way (USPS flat rate shipping used for the purposes of example...FedEx/UPS won't be much different): you put four glasses into a USPS flat rate shipping container. Shipping costs about $8. If you only put two glasses into the same container, shipping still costs the same $8. So, if four glasses cost $24 with shipping, each glass alone costs $4. It follows, then, that two glasses with shipping should cost $16, not $12.
Sorry, bad coffee...figured I'd pipe up in your defense. Didn't mean to thread-jack.