I'm new to Zen cart, so be gentle with me.

I have just managed to configure my shipping costs by individual items by checking in the FAQs ( very helpful )
What i'd now like to do is offer free shipping on some of my smaller items in stock ,when ordered in conjunction with a larger item.

EG : polo top priced at $30 + $20 shipping
Keyring priced at $5 + $2.50 shipping

Now i need to make it that they are only paying shipping on the larger item
any help will be appreciated