Let's say that you see someone doing a 50 $ job on a neighbor's house.
You ask; "How much to do that on my house?"
The Service Provider answers "When?"
You say :
1. "Next week." He replies: "50$."
2. "When you finish here." He replies: "35$."
A 50$ job is really a 35$ job with a 15$ service call charge just to come out to the site. So, if he has to come back out there another day, you have to pay the same charge. Now, if you catch him immediately afterwards, your neighbor has already paid that service charge! He doesn't have to come back out there again.