This is a brilliant idea, and a bargain too! This Hot Wheels Rewards Car Diecast Vehicle 10-Pack lets kids collect cars as rewards for a job well done, and it's down to only £10 at Amazon.
The Hot Wheels Rewards Car packs are a great way to encourage good behaviour and reward achievements, with 10 individually wrapped 1:64 scale Hot Wheels vehicles. Each car is wrapped in a 'blind bag' so the car is a surprise they get as their reward.
Also included in each bag is a gold sticker that can be added to the included reward chart, so they can track their accomplishments.
You could use this for all sorts of things, from tidying their room, to doing well at school, or whatever other kind of behaviours you want to encourage and recognise.
If you have Amazon Prime there's free One Day Delivery. Non-members get free delivery on orders of £20 or more, so grab a couple of these (as they're random mixed assortments) and you'll qualify for free delivery.