Remember that when you sign for the tip.

[Agito a42]

Source: Die Hard (1988), an American movie.

It's Christmas Eve. John, who lives in New York, arrives at Los Angeles to visit his estranged wife, Holly. At the airport John is met by a limousine driver, Argyle, charged with taking him to the company where she works. During the ride, it becomes clear that John's here to make up with his wife. So, as Argyle drops him off at the entrance, he tells him that he'll pull in the parking garage and be waiting for his call, in case it doesn't work out, so he could take him to a hotel.
John: You're all right, Argyle.
Argyle: Just remember that when you sign for the tip.

Could you please explain to me when John is supposed to sign for the tip? If Argyle leaves, they'll probably never meet again, right? Will the company Argyle works for send John a bill or something? Where to? New York? How soon Argyle receives his tip? This procedure is absolutely obscure to me.

 

[PaulQ]

This is not really a language question: It seems that the bill for the cost of the limousine will sent to John.
1. The bill may have the tip shown on it. John has to pay the whole bill and sign the bill.
2. There may be a blank where the tip is mentioned, and perhaps John will have to write in an amount... who's to say?

 

[Agito a42]

I see. Thank you, PaulQ.

 

[Orble]

I agree with PaulQ. “Sign for the tip” has no idiomatic resonance for me.

(Perhaps you should wait for an American opinion though. Tipping is much more of “a thing” there than in Australia and they do have some unique expressions around it.)