func json(from dict: [String: Any]) -> String {
  let jsonData = try! JSONSerialization.data(withJSONObject: dict, options: [])
  let str = String(data: jsonData, encoding: .utf8)!
  return str
}

struct Product {
  var price: Double
  var quantity: Int

  var toJSON: String {
    return json(from: ["price": price, "quantity": quantity])
  }
}

let p1 = Product(price: 0.90, quantity: 3)
print(p1.toJSON) // {"price":0.90000000000000002,"quantity":3}
let p2 = Product(price: 0.99, quantity: 3)
print(p2.toJSON) // {"price":0.98999999999999999,"quantity":3}

extension Product {
  var toRoundedJSON: String {
    // Use a NSDecimalNumber, rounded with a NSDecimalNumberHandler, and use that in the dict instead of a Double
    let currencyBehavior = NSDecimalNumberHandler(roundingMode: .plain, scale: 2,
                                                  raiseOnExactness: false, raiseOnOverflow: false, raiseOnUnderflow: false, raiseOnDivideByZero: true)
    let formattedPriceDN = NSDecimalNumber(value: price).rounding(accordingToBehavior: currencyBehavior)

    return json(from: ["price": formattedPriceDN, "quantity": quantity])
  }
}

print(p1.toRoundedJSON) // {"price":0.9,"quantity":3}
print(p2.toRoundedJSON) // {"price":0.99,"quantity":3}