Yes, you have chosen the most relevant rule of law.  Remember, the issue in this case is:

In the absence of an agreed time for payment, when must a buyer of goods pay the purchase price?

The rule of law you selected is:

(c) Delivery of goods bought and sold, and payment for those goods, should take place at the same time unless otherwise agreed.

Clearly, this rule provides the answer to the question raised.

In more complex case studies, more than a single rule of law may be involved, but the principles of selecting them remains the same.