Ship: Alone = 1price, with others = 2price. Is this possible?

Okay,

the thread title may be a bit confusing but hopefully not so that no one is able to make any suggestions. Here's what I am hoping to do:

I am using another cart that allows this but I want to use ZC instead.

Currently, I sell clothing and accessories (t-shirts, hoodies, necklaces, wristbands and so on) and have my shipping set up as such - each item individually has a flat rate for shipping. I only ship to addresses within the U.S. For example, one t-shirt shipped by itself ships USPS Priority for a flat rate of $5.00. Should the customer want to add another shirt to their order, the next shirt adds another $2.50 to the shipping cost. All of the items have a 'shipped alone' & 'shipped with another item' rate. Can I do this with ZC?

Examples:
T-shirt >> Shipped alone = $5.00 | Shipped with others = $2.50
Necklace >> Shipped alone = $3.00 | Shipped with others = $2.00

and so on...

Thanks!

You'd have to write your own shipping module logic, but yes, it is possible.