Have you read this:
I have not tried to do any traffic shaping myself, but this short article seems to make it seem pretty simple.
As far as I can tell, you will have to configure a virtual device and associate it to your NIC card. Once you have done this, you can set traffic limits on that interface. After that you can either route traffic through that interface by updateing your routing tables, or configure iptables to use that virtual interface in your ruleset. Give it a read and let me know if it clears things up a bit.