The volunteer who asks for supplies is trying to make the promised work possible.

Requests for materials can sound like complaints, especially to whoever is watching the budget. Almost always they are the opposite: someone taking the commitment seriously enough to work out what it actually requires. Refusing the small cost while expecting the result asks a person to make up the difference personally, which they will do once or twice and then quietly stop.

If someone helping you asks for something, treat it as a sign of seriousness rather than a demand. Get them what they need, or say honestly what is not available so the plan can shrink to fit.