First thought that comes to mind: Look into how Virtual Assistant services do pricing... Whatever it is, I'm sure it could be improved, but it might be a good starting point. I imagine they either charge a % of the cost (eg, milk+courier+0.15(milk+courier), or a subscription (plus cost of each transaction of course), or time-based (how long it took to complete the request). Maybe urgency-based?

Or a combination of any of above.