Some mission need room and lift. Room is easy but for the weight make research a combo of motor, wings lift and tought wheels.

V tail is effective and less expensive (only 2 tailwings instead of 3). Rear wheel can be forget at start either to save money as the game is very permissive on take off and landing.

Missions requiring speed are the most difficult but affordable.

Tailor your aircraft to the load and don't hesitate to put everything into the engines. Weak structures can cause problems. You can adjust the engine in the editor to avoid going too fast (right-click on the motor and clic on the gear).

https://images.steamusercontent.com/ugc/20937484299878101/176C27165BEF20E82F95B520F9F6F043059F269F/?imw=256&&ima=fit&impolicy=Letterbox&imcolor=%23000000&letterbox=false

You could name your plane with his loading room number to find rapidly the most appropriate one : "Fast 150" or "Heavy 400"

Last, crashing on a airfield sometimes gives you delivery completion ;) (until next patch...)

Nice builds and Have fun !