All right, 6/6 quests done.
Some general advice for getting Gold Medal on the later levels.
1) Helicopters are crap. They cost SIGNIFICANTLY more than trucks, and are not produced from a factory any faster. Since helicopter deliveries also cost more (and thus return lower profit), this leads to helicopters being completely pointless on any level that doesn't involve water. Their greater speed is completely pointless because it doesn't actually GET you anything. I have 3 starred all levels easily with trucks - INCLUDING the final level, and I did not buy a single helicopter except for the level that straight up requires it (when your starting factory is on an island).
2) Do not buy new plants unless you have enough trucks to actually send out from them. If you are at 0 trucks for your current number of plants, it means you need to buy at least 3 more trucks before you get another plant.
Level 24 is the only level where I bought a 2nd factory inside the first minute of play, all others it is generally better to build up trucks and a bit of a bank before you build a 2nd factory.
3) Make sure your new plants are actually in a position to get you more profit. Later levels have a significant number of locations for plants (far more than you are going to build), ideally a new plant is on the other side of the level to make much faster deliveries. It isn't very useful to build a new plant right next to a current plant unless you are very good at hitting the hotkeys.
4) Large orders are MUCH more profitable than small orders (even though the small orders have individually higher profit). This is because the math for the order finish bonus goes up massively the higher it gets. If you read the game information, the bonus for finishing a delivery is calculated as ORDER SIZE^2 + ORDER SIZE^2 * (% PATIENCE).
So, for instance, the bonus for finishing an order of 4 is only $32. (4*4 + 4*4*1.00) Meanwhile, the bonus for completing a 16 order can be as much as $512 - (16*16) + (16*16)*1.00. And every dollar of that bonus is pure profit. So completing 4 orders of 4 only gets you a total bonus of $128, while completing that 1 order of 16 gets you a bonus of $512. Obviously you can see the difference here.
5) ENEMY DELIVERIES SUBTRACT FROM THE ORDER BONUS TOTAL. Unlike the previous game, where you got the bonus for the full order as long as you tagged the last delivery, in this game your bonus is calculated based ONLY on your deliveries. So if a 16 order pops, and you let an enemy fill 10 of them, you only get a bonus of $72 for finishing 6, rather than $512 for finishing 16.
That means that you need to have money available to fully tag big orders as soon as they spawn so they can't cut into your bonus. So yes, you might have enough to build a new plant, but if that consumes all of your money then you'll lose money off big orders until you build your bank up again. So you need to carefully balance building new plants/trucks and making sure you have enough of a bank to immediately fill big orders.
6) When time ends, any trucks ALREADY DEPLOYED will finish deliveries. This means you don't have to worry about the distance from factory to target at the end. As long as the truck is on the map it will finish the delivery, regardless of distance. Which is one more reason helicopters are useless.
7) To elaborate on #6, understand the plant hotkeys. Normally it defaults to the closest factory when you are clicking on an order. However, if you hold '1' '2' '3', etc, while you are hovering orders, it will change the plant priority to that specific plant. Don't do this normally, as it generally means trucks have a much higher return time, but it is EXTREMELY useful at the end of a level to finish off big orders to get that big bonus.
i.e. if you have 30 seconds left and a 15 order pops up, you cannot complete it with 1 plant. However, if you hit 5 deliveries from 3 different plants, they will deploy 5 trucks each and finish that order for the big $450 bonus right at the end.
That's it for basic tips. I've gotten Gold Medals on all stages so if you have trouble with specific stages feel free to ask.