How to Build a Custom Coupon and Discount System in FlutterFlow

In Firestore, create a coupons collection with fields: code (String, uppercase, unique), type (String: 'percentage' or 'fixed'), value (Double: e.g., 15.0 for 15% or 10.0 for $10 off), minOrderAmount (Double: minimum cart total required), maxUses (Integer: total redemptions allowed, 0 for unlimited), currentUses (Integer, default 0), expiresAt (Timestamp), isActive (Boolean, default true), and applicableCategories (List of Strings, empty means all categories). Set Firestore rules: read allowed for authenticated users (needed for admin panel), write restricted to admin role users only. The code field must be stored uppercase and matched uppercase during validation.

On your checkout page, add a Row below the order summary containing a TextField (hint text: 'Enter coupon code', text capitalization: Characters) and a Button labeled 'Apply'. Add three Page State variables: appliedCouponCode (String), discountAmount (Double, default 0), and couponError (String). Below the Row, add a Text widget for couponError (red color, Conditional Visibility: couponError is not empty) and a Container showing the applied discount (green background, showing 'Coupon SAVE20 applied: -$15.00', Conditional Visibility: discountAmount > 0). Include a small 'Remove' IconButton in the discount Container that resets all three Page State variables to their defaults. Update the order total Text to subtract discountAmount from the cart subtotal.

Create a callable Cloud Function named validateCoupon that receives the coupon code and the current order total. The function performs these checks in order: (1) look up the coupon document by code (document ID), return error if not found; (2) check isActive is true; (3) check expiresAt is in the future; (4) check currentUses < maxUses (or maxUses == 0 for unlimited); (5) check order total >= minOrderAmount. If all checks pass, calculate the discount: for percentage type, multiply order total by value/100 and cap at the order total; for fixed type, use the value directly but cap at the order total. Return the discount amount and coupon details. This server-side validation prevents users from bypassing checks by manipulating the client.

1// Cloud Function: validateCoupon
2const functions = require('firebase-functions');
3const admin = require('firebase-admin');
4
5exports.validateCoupon = functions.https.onCall(async (data, context) => {
6  const { code, orderTotal } = data;
7  if (!context.auth) throw new functions.https.HttpsError('unauthenticated', 'Login required');
8
9  const couponRef = admin.firestore().collection('coupons').doc(code.toUpperCase());
10  const couponSnap = await couponRef.get();
11
12  if (!couponSnap.exists) throw new functions.https.HttpsError('not-found', 'Invalid coupon code');
13
14  const coupon = couponSnap.data();
15  if (!coupon.isActive) throw new functions.https.HttpsError('failed-precondition', 'Coupon is no longer active');
16  if (coupon.expiresAt.toDate() < new Date()) throw new functions.https.HttpsError('failed-precondition', 'Coupon has expired');
17  if (coupon.maxUses > 0 && coupon.currentUses >= coupon.maxUses) throw new functions.https.HttpsError('resource-exhausted', 'Coupon usage limit reached');
18  if (orderTotal < coupon.minOrderAmount) throw new functions.https.HttpsError('failed-precondition', `Minimum order $${coupon.minOrderAmount} required`);
19
20  let discount = coupon.type === 'percentage'
21    ? Math.min(orderTotal * (coupon.value / 100), orderTotal)
22    : Math.min(coupon.value, orderTotal);
23
24  discount = Math.round(discount * 100) / 100;
25  return { discount, type: coupon.type, value: coupon.value, code: code.toUpperCase() };
26});

On the Apply button's On Tap Action Flow: first, convert the TextField value to uppercase. Then call the validateCoupon Cloud Function using a Backend Call action, passing the coupon code and the current cart subtotal. On success: set Page State appliedCouponCode to the returned code, discountAmount to the returned discount value, and clear couponError. On error: set couponError to the error message from the Cloud Function response and clear discountAmount. Update the order total display: bind it to a Custom Function that calculates subtotal minus discountAmount. On successful order completion (after Stripe Checkout or payment), call a second action to increment the coupon's currentUses by 1 using a Cloud Function or direct Firestore Update Document with FieldValue.increment(1).

Create an AdminCouponsPage restricted to admin users (check user role on page load). Add a ListView bound to a Backend Query on the coupons collection ordered by expiresAt descending. Each row shows: code (bold), type badge (percentage or fixed), value, usage as 'currentUses / maxUses', expiresAt formatted date, and an isActive toggle Switch. The Switch On Change action updates the coupon's isActive field. Add a FloatingActionButton that opens a BottomSheet form for creating new coupons with fields: code TextField, type DropDown (percentage/fixed), value TextField (number keyboard), minOrderAmount TextField, maxUses TextField, expiresAt DateTimePicker, and applicableCategories ChoiceChips. The Save button creates the coupon document using the uppercase code as the document ID.