Welcome back to the categories of contract language! Consider the following alternatives:
Language of Discretion
The Customer may purchase Widgets only by using Acme’s purchase-ordering system to submit a purchase order to Acme.
Language of Obligation Used to Express a Condition
To purchase Widgets, the Customer must submit purchase orders to Acme through Acme’s purchase-ordering system.
Which do you prefer? I prefer the second alternative. Using instead language of discretion would suggest the possibility of breach if the customer were to purchase widgets any other way. That doesn’t make sense: there is no other way to effect a purchase.
