What a cute concept! Good play on the theme, with the letters connecting people mailbox to mailbox. The arrow was always pointing me in the right direction, and it being positioned with under the timer helped immediately show me I was in a time constraint so very good UI there. The controls worked as advertised, I would only advise to make the driving a bit less wild. In driving games it's important to make the driving the central part of the gameplay even if the story is elsewhere (focused here on delivering letters). If the driving doesn't feel 'good', it can make things frustrating for the player. The truck overshooting when trying to turn made it difficult to handle, and with the addition of the timer sometimes made it feel like the game was trying to make me fail actively rather than allowing me to fail on my own.
Don't make the gameplay the player's main concern, make the game mechanics(getting the letters, reaching the right house)where the difficulty comes from. I'd simply adjust how wide the truck can turn or go past a place once direction is changed.