Read the conversion first: it says what exists once a checkout is paid, and which record to change from then on, and every later page builds on it. The unit then follows the purchase in the order it happens. The money comes first: invoices as the commercial document, transactions as the money itself, and refunds, whose adjustment is meant to go negative. Then the goods: stock kept in step with the system that ships them, fulfilments and shipments with the status a customer sees, and the two order screens of the member area.
The unit ends with the customer's emails: which record and event each one hangs on, and what its template reads. The mechanism behind them, a job with a Send Email action, is Guide 9's, and the pages link to it rather than repeat it. Every page says which currency an amount is in and which language a name is in, for stores that sell in more than one of each.