Make an option to note a referral source for each purchase?

Customers may be initially referred by one source, and become a frequent customer.

However, frequent customers may be drawn in with online promotions, and it would help to be able to track that as well.

This could cause each individual sale to be a different referral source, but there's no way to note this each sale, unless you make the referral source a line item in the invoice--which is more convoluted when attempting to track each purchase.