I think you will lose alot of customers by having to mail it to them. It is an impulse buy and they want to put it in that night, they wont even watch it if it gets there after halloween. I have tried something similar with poor results.
I want to note though that all you have to lose is the money for the signs offering the service. If you already have the cameras and are setting them up then they are not a cost factor. If you were going to buy cameras just for this service then I would not do it.