Not sure about whether they need to be processed as payroll or payables, but if you want to issue t4's or ROEs for them through sage it would need to go through the payroll module

It may vary depending on the province. In BC unless the income is from self-employment (the worker has some autonomy over hours of work, tasks, etc.) it should almost always be processed through payroll to be on the safe side; however you can always contact CRA for a ruling if unsure.
Based on the small dollar value the income would likely only be subject to EI deduction at 1.88%. If the company wants to continue giving the volunteers $100.00 exactly and still go through payroll they would just record a slightly higher gross amount (101.92) to net 100.00.