Rules that matter more than the wording
Send from your own name and address, not "billing" or your payment processor. Say the amount. Give exactly one link, and make it the Stripe hosted invoice page for that invoice, so the subscriber updates the card and pays in the same place. Keep it under a hundred words. Never threaten, and never imply they are at fault; most of the time an expired card is the whole story.
Email 1: right after the failure
Subject: Your $8 payment to Jane's Field Notes didn't go through
Hi Sam, We couldn't process the $8.00 payment for your Jane's Field Notes subscription. Your subscription stays active for now, but we'll need an updated payment method to keep it that way. Update your payment method and pay now: [link] If you've already resolved this, you can ignore this email. Reply to this message if you have any questions. Jane
Email 2: three days later, still unpaid
Subject: Still need a working card for Jane's Field Notes
Hi Sam, Quick nudge: the $8.00 renewal for Jane's Field Notes is still unpaid. Cards expire and banks block things for no reason, so this is usually a thirty-second fix. Update your card here: [link] If you'd rather stop your subscription, that's completely fine too; just let me know by replying and I'll take care of it. Jane
Email 3: the last reminder
Subject: Last note about your Jane's Field Notes subscription
Hi Sam, This is the last reminder I'll send. Your Jane's Field Notes subscription is still unpaid, and if it stays that way it will end on its own soon. If you'd like to keep it, update your card here: [link] Either way, thank you for being a subscriber. Jane
What to avoid
Logos, banners and marketing layouts make it look like a promotion and get skimmed. "Action required" subject lines read like phishing. Sending from a no-reply address removes the one thing that makes these work, which is that a real person can be answered. And sending a reminder after the invoice was paid is the fastest way to annoy a loyal subscriber, so check the invoice status before every send.
Timing
First email within minutes of the failure, while the subscriber still remembers what they subscribed to. Follow up every three days, and stop after about thirty days; by then Stripe's retries have run out and the subscription will typically cancel on its own. Stop instantly when the invoice is paid or the subscription is cancelled.