Checkout: Change plan upsell calculation to take volume and discounts…

… into account (#90893)

* Change savings calculation to take volume discounts into account

* Add percentage and first_unit_only to cart item cost_overrides

Also make it clear that `cost_overrides` is always set.

* Only use undiscounted domain price in calc for first year discounts

* Re-apply coupon code discount to price difference

* Add cost_overrides to empty response cart

client/my-sites/checkout/cart/cart-free-user-plan-upsell.tsx

@@ -36,6 +36,10 @@ const isRegistrationOrTransfer = ( item: ResponseCartProduct ) => {
 	return isDomainRegistration( item ) || isDomainTransfer( item );
 };
 
+function applyPercentageDiscount( percentageDiscount: number, basePrice: number ): number {
+	return basePrice - basePrice * ( percentageDiscount / 100 );
+}
+
 function UpgradeText( {
 	planPrice,
 	planName,
@@ -46,9 +50,43 @@ function UpgradeText( {
 	firstDomain: ResponseCartProduct;
 } ) {
 	const translate = useTranslate();
+	// Use the subtotal with discounts for the current cart item price if the
+	// item's discounts are only for the first year, but use the subtotal
+	// without discounts to calculate the price after the upsell because the
+	// upsell will remove other first year discounts.
+	//
+	// For example, if the domain has a sale coupon then it will be removed by
+	// the "first year free" bundle discount when a plan is added to the cart
+	// because the bundle discount of 100% is higher than the sale coupon
+	// discount. Since we are trying to compare the current cart price with the
+	// predicted cart price with the plan, we must consider the current cart
+	// with the sale and the predicted cart without the sale.
+	//
+	// This will not be accurate for all types of discounts and predicting what
+	// discounts will be applied is difficult, but this makes an attempt based
+	// on what discounts are currently applied.
+	const isDomainDiscountFromFirstYearOverride = firstDomain.cost_overrides.every(
+		( override ) => override.first_unit_only
+	);
+	// Coupon code discounts will apply before and after the upsell, so we must
+	// re-apply them to the price difference also.
+	const domainCouponCodeDiscount = firstDomain.cost_overrides.find(
+		( override ) => override.override_code === 'coupon-discount'
+	);
+	const domainCouponPercentageDiscount = domainCouponCodeDiscount?.percentage ?? 0;
+	const domainInCartPrice = isDomainDiscountFromFirstYearOverride
+		? firstDomain.item_subtotal_integer
+		: firstDomain.item_original_subtotal_integer;
+	const domainInCartPricePerYear = firstDomain.item_original_subtotal_integer / firstDomain.volume;
+	const cartPriceWithDomainAndPlan =
+		domainInCartPricePerYear * ( firstDomain.volume - 1 ) + planPrice;
 
-	if ( planPrice > firstDomain.item_subtotal_integer ) {
-		const extraToPay = planPrice - firstDomain.item_subtotal_integer;
+	if ( cartPriceWithDomainAndPlan > domainInCartPrice ) {
+		// Re-apply any coupon code discount.
+		const extraToPay = applyPercentageDiscount(
+			domainCouponPercentageDiscount,
+			cartPriceWithDomainAndPlan - domainInCartPrice
+		);
 		return translate(
 			'Pay an {{strong}}extra %(extraToPay)s{{/strong}} for our %(planName)s plan, and get access to all its features, plus the first year of your domain for free.',
 			{
@@ -66,8 +104,12 @@ function UpgradeText( {
 		);
 	}
 
-	if ( planPrice < firstDomain.item_subtotal_integer ) {
-		const savings = firstDomain.item_subtotal_integer - planPrice;
+	if ( cartPriceWithDomainAndPlan < domainInCartPrice ) {
+		// Re-apply any coupon code discount.
+		const savings = applyPercentageDiscount(
+			domainCouponPercentageDiscount,
+			domainInCartPrice - cartPriceWithDomainAndPlan
+		);
 		return translate(
 			'{{strong}}Save %(savings)s{{/strong}} when you purchase a WordPress.com %(planName)s plan instead — your domain comes free for a year.',
 			{

packages/shopping-cart/src/empty-carts.ts

@@ -58,5 +58,6 @@ export function getEmptyResponseCartProduct(): ResponseCartProduct {
 		product_type: 'test',
 		included_domain_purchase_amount: 0,
 		product_variants: [],
+		cost_overrides: [],
 	};
 }

packages/shopping-cart/src/types.ts

@@ -418,7 +418,7 @@ export interface ResponseCartProduct {
 	 * The override_code is a string that identifies the reason for the override.
 	 * When displaying the reason to the customer, use the human_readable_reason.
 	 */
-	cost_overrides?: ResponseCartCostOverride[];
+	cost_overrides: ResponseCartCostOverride[];
 
 	/**
 	 * If set, is used to transform the usage/quantity of units used to derive the number of units
@@ -525,6 +525,8 @@ export interface ResponseCartCostOverride {
 	old_subtotal_integer: number;
 	override_code: string;
 	does_override_original_cost: boolean;
+	percentage: number;
+	first_unit_only: boolean;
 }
 
 export type IntroductoryOfferUnit = 'day' | 'week' | 'month' | 'year' | 'indefinite';