tipping question

[Wynton]

I have a vague recollection that we used to have a lot (too many) threads here about tipping, but I have a specific question.

Today, movers are bringing a baby grand piano to my house. The company that sold me the piano pays for the moving cost.

In addition, we are going to ask the movers to move an existing upright piano to another floor (until we get rid of it). They told me there will be a fee for that.

So, the question is simply: in addition to the extra fee to move the second piano, should these guys be tipped? And how much?