Hi Beejoy,
It seems I was mistaken about a schema for USA - nothing is available yet. But the important thing is to gain experience with Aggregation in your system. All of the existing schemas try to combine items with the same Tariff Code (Schedule B number as you call it), and I expect you'd get good results. Typically, the schemas also take into account many other fields, such as item currency, Country of Origin, Customs Procedure Code, and so on.
Regards,
Dave