Need shipping help

Hi eveyone. I'm new to zencart and am trying to set up shipping.

I sale table runners. I usually ship my items in a flat rate envelope. I know I can fit 6 runners in a flat rate envelope, 15 runner in a large flat rate box. How do I set it up so that when a customer orders say...9 runners it knows to charge them $10.95 for a flat rate box but on the flip side if a customer orders say 8 runners it knows to charge them $4.95?

Any thoughts on how to se this up? Thanks

More Info
Im running Zen cart Version: Zen Cart v1.3.8a/v1.3.8
No upgrades fresh install last week
No add ons that I know of

url: http://kennedycouture.com

Would Table Rate table shipping work for you?

Set to Item count and use something like:
8:4.95, 15:10.95

But what happens when the number of runners is above 15? :unsure: