How do I make sure they will drop me off at Allegiant departure terminal at LAX？

LAX Terminal 5 - Instruct your driver

Your driver should be aware of where to drop you off as long as you communicate to him or her that you are departing on an Allegiant flight.  (With Uber or Lyft, you could drop the pin on an exact terminal, but the driver will most likely as you as you approach the airport.)

Allegiant Airlines' terminal has moved in 2017 to Terminal 5.  There's more information here:

Beginning May 13, 2017 Allegiant flights will depart/arrive at Terminal 5.

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Uber Driver, CEO of TaxiFareFinder and Unleashed, LLC