It varies a lot since I/E gives you a good amount of time to do whatever you want during the export cooldown. But here are two possibilities:
■ if you haven't filled your warehouse with all 32 different cars yet:
in your office, go to the computer on your desk, start a sourcing mission
go get the car, bring it to your warehouse
if you don't already have a top range car in your warehouse, go back to your office, source another car and repeat.
if you already have a top range car in your warehouse, go to the warehouse computer in the office space and start a delivery mission for it
deliver the car; the 20m export cooldown starts as soon as the export mission is completed
go back to your office, start a sourcing mission; you will have enough time to do two or three of them before the export cooldown wears off and you can export another car.
■ once you have all 32 different cars in your warehouse, it's more straightforward:
in your warehouse, go to the computer and start a delivery mission for a top range car
deliver the car; the 20m export cooldown starts as soon as the export mission is completed
go back to your office, source a car, bring it to the warehouse
do whatever you want to do during the 10-15 minutes left on the export cooldown
when the cooldown is over, start again with a top range delivery.
If you want to mix VIP jobs with I/E to make more money, you can do something like this to avoid being slowed down by mission cooldowns:
That way, you're doing three missions doing the 20 minutes export cooldown (two VIP missions + sourcing one car), and you're always doing an I/E mission while the cooldown on regular VIP missions is ticking off, so you're earning an extra ~$40k ($20k per VIP mission) during each export cooldown.