If you spend your personal money on volunteer activities, you can expect to be reimbursed
There lies a point unknown somewhere between $1.50 and $150 above which you should get prior approval before spending money you expect to have reimbursed, but we can't easily define where that is.
Factors like urgency, how much experience you have volunteering, what the expense is for, how large the amount is, etc. will affect this. You shouldn't rent a helicopter and ask us to reimburse you. But neither should you avoid buying a needed tube of construction adhesive, a few bolts, a bit of rope for your project.
If you're uncomfortable with this ambiguity, any board member can authorize you to spend money. We rely upon board members to decide when they need to request a group decision.
But sometimes there is no board member handy, and you have to decide. The lower the number or the greater the urgency, the more you should just go ahead
You can claim reimbursement with this form.