At any time during the game when you have a vehicle that needs repairing or more ammo you can get both for free by doing a mission and canceling the contract. All you need to do is park the vehicle in the base where the mission is, accept the contract, and then cancel the contract. You'll get the retry screen, select no, and then your vehicle is fully repaired and rearmed without losing money. You can then do the same mission at a later time.