Discounts
A customer who has already bought for over € 1000, gets a discount of 10% on the whole order.
For every product of category "Switches" (id 2), when you buy five, you get a sixth for free.
If you buy two or more products of category "Tools" (id 1), you get a 20% discount on the cheapest product.
Todo
Add domain and presentation models to replace arrays, rate limiter, auth, cache.