[ic] Charging sales tax to billing zip?

[interchange-users@interchange.redhat.com (Rick Shank)]

This is how Pennsylvania works at least....the sales tax gets charged to 
according to whether the purchaser is in PA or not.

How can I get this to work, such that it still allows you to not provide a 
separate billing address if it's the same as the shipping (how it works 
out of the box), so in this case it would calculate sales tax based on 
zip.   However, if you fill in b_zip , it will calculate based on this?

Thank you,

~Rick Shank