group sets of transactions per customer

In our business we have an initial order and we then will have potentially years of monthly orders all related to the original order.

For example, let's say we sell WidgetA to CustomB, we will then sell a WidgetC every month and a WidgetD once a year.

We would like to keep track of all these related orders and be able to group them somehow.

I can think of a couple ways to accomplish this:
[LIST=1]
[*]Add a Transaction field to the Transaction record, "Original Transaction" that would be set to the original order for all subsequent orders.  This seems like a good solution to me.