Find Bug in Swift Snippet

Rakuten's shopping app checkout screen applies a loyalty discount and lets shoppers redeem Rakuten Points before totalling the order. A mobile engineer on the checkout team wrote the function below, and QA is reporting totals that do not match what the receipt should show.

struct CartItem {
    let sku: String
    let priceYen: Int
    let quantity: Int
}

func checkoutTotal(items: [CartItem], pointsAvailable: Int, prices: [String: Int]) -> Int {
    var subtotal = 0
    for item in items {
        let unitPrice = prices[item.sku]!
        subtotal += unitPrice * item.quantity
    }

    let discountPercent = 10
    let discountedTotal = subtotal - (subtotal * discountPercent / 100)

    var pointsUsed = pointsAvailable
    if pointsUsed > discountedTotal {
        pointsUsed = discountedTotal
    }

    var finalTotal = 0
    for item in items {
        finalTotal += item.priceYen * item.quantity
    }
    finalTotal -= pointsUsed

    let shippingFee = subtotal > 3000 ? 0 : 500

    return finalTotal + shippingFee
}

Find the bugs in this Swift snippet and explain what each one causes at checkout.