Redeeming a discount code at membership checkout

When a supporter joins a membership tier, they can apply a discount code right on the join surface before they pay — whether they reach it from a Builder membership block, a members-only paywall, or the standalone /buy/sub link.

To redeem a code at checkout:
1. On the membership join panel, find the Discount code field
2. Type the code and press Apply
3. If the code is valid for this membership, the panel shows the savings and the adjusted first-payment total (e.g. "Code FOUNDER50 applied · 50% off, every renewal")
4. Continue to payment — the card step charges the discounted amount

Good to know:
• The code is re-validated server-side when the checkout session is minted, so the price the buyer pays is always the authoritative one — a tampered client can't force a bigger discount.
• Recurring cadence carries through: a "first payment only" code discounts just the join charge, while "every renewal" or "first N months" codes keep discounting future renewals automatically.
• A gift card on file can't be combined with a recurring membership discount — the buyer applies one or the other.
• If a code is expired, disabled, exhausted, or scoped to a different tier, the field shows a plain-language reason and no discount is applied.