Add Discount Functionality

[heywinit]

I came across the project on X today. I could really use this for an upcoming project I have. The only thing I'm currently missing is discounts, and I would like to add those here. I've written down a basic plan on what I'm looking forward to adding. Let me know if you'd like any changes.

Aim: Add support for discount codes for carts (percentage, fixed amount, free shipping).

API

• Admin: GET|POST|PATCH|DELETE /v1/discounts
• Public: POST /v1/carts/:id/apply-discount, DELETE /v1/carts/:id/discount

Discount flow

1. Discount validation (active, dates, limits, min purchase)
2. Calculating discount on subtotal of the cart
3. Update cart totals to include discount
4. Integrate with Stripe Checkout (allow_promotion_codes)
5. Store discount in orders via webhook
6. Tracking discount code usage

Types of discount

• Relative: 20% off, max $50
• Absolute: $10 off orders over $50
• Free shipping: free shipping on orders over $25

DB changes

• Adding two new tables, discounts and discounts_usage

discounts table
- id – text, primary key
- store_id – text, not null, foreign key → stores(id)
- code – text, unique
- type – text, not null, enum (percentage, fixed_amount, free_shipping)
- value – integer, not null
- status – text, not null, default active, enum (active, inactive)
- min_purchase_cents – integer, default 0
- max_discount_cents – integer, nullable
- starts_at – text, nullable
- expires_at – text, nullable
- usage_limit – integer, nullable
- usage_count – integer, not null, default 0
- stripe_coupon_id – text, nullable
- stripe_promotion_code_id – text, nullable
- created_at – text, not null, default now
- updated_at – text, not null, default now

discount_usage table
- id – text, primary key
- discount_id – text, not null, foreign key → discounts(id)
- order_id – text, not null, foreign key → orders(id)
- customer_email – text, not null
- discount_amount_cents – integer, not null
- created_at – text, not null, default now

• Changes to existing tables

-- carts
ALTER TABLE carts ADD COLUMN discount_code TEXT;
ALTER TABLE carts ADD COLUMN discount_id TEXT REFERENCES discounts(id);
ALTER TABLE carts ADD COLUMN discount_amount_cents INTEGER DEFAULT 0;

-- orders
ALTER TABLE orders ADD COLUMN discount_code TEXT;
ALTER TABLE orders ADD COLUMN discount_id TEXT REFERENCES discounts(id);
ALTER TABLE orders ADD COLUMN discount_amount_cents INTEGER DEFAULT 0;

Misc

• Single discount per cart. Validate on cart update and checkout.

[stritt]

Great plan! A couple questions before you start:

1. Are you going to use Stripe's native allow_promotion_codes, or apply discounts as line item adjustments? I'd lean toward the latter so Merchant stays the source of truth.

2. How are you thinking about free shipping working given shipping comes from Stripe Checkout? Zero out tge shipping options, or treat it as a fixed rebate?

3. Could you add the proposed table/column changes to the plan?

4. Are you going to update the Admin Panel? Or would you want me to? Either way is fine it's still pretty early working through some rough edges.

Otherwise looks solid. Looking forward to the PR!

[heywinit]

1. Sure I was also thinking of applying discounts as line item adjustments. I thought you might prefer Stripe.

2. For the free shipping, I prefer treating it as a fixed rebate.

3. I had the new tables in the plan prior to this. Added the column modifications for carts and orders tables as well.

4. I'll look into the admin panel once I'm done implementing this. I'll be able to propose something better once I have a base ready.

[adlonymous]

1. Makes sense to apply discounts as line item adjustments to keep the merchant as source of truth, in case you want to use alternative payment solutions other than Stripe

[heywinit]

From what I could find, Stripe doesn't support negative line items in a checkout flow. @stritt do you have any other suggestions?

[heywinit]

#2

Here's my draft PR

[heywinit]

@stritt bump!

[stritt]

overall it looks great, and you're right it looks like stripe checkout doesnt support negative line items :(

i think the cleanest path forward is to sync discounts into stripe as coupons or promo codes. you thought ahead and your schema already has stripe ids, so when a discount is created in merchant we create the matching object in stripe and store the id. then we attach it to the checkout session directly.

merchant stays the source of truth for all validation and usage tracking. stripe is just handling how the discount shows up in checkout.

for free shipping, we can either use stripe’s shipping-specific coupons or just select a $0 shipping rate when the discount is applied.

this keeps things flexible and wont lock us into stripe.

happy to talk it through more if helpful!

[heywinit]

that sounds good. i'll add it.

[heywinit]

i added a scheduled sync for reconciling with stripe for the discount codes in the latest commit on the same PR

[heywinit]

@stritt should I create a new issue for the admin panel or does this work?