Which of the following is a method of purchasing commissary?

Payroll deduction is a valid method of purchasing commissary because it allows for the cost of items to be automatically deducted from an employee's paycheck. This system is often utilized in various organizations, including those involved in interagency incidents, to facilitate purchases in a way that is convenient and ensures prompt payments.

Using payroll deduction can help streamline the transaction process, making it simpler for both the employee and the organization. It reduces the need for cash transactions or other forms of payment at the time of purchase, providing a reliable accounting method for managing these sales.

In contrast, while gift cards, cheques, and online transfers are commonly used payment methods in many settings, they do not represent a systematic approach within the context of commissary purchases as effectively as payroll deductions do. Gift cards might require advance purchase, cheques involve manual processing, and online transfers could introduce delays depending on the payment systems in place.