insufficient_funds
Insufficient funds. The card doesn't have enough balance or available credit to cover the charge.
Will retrying the charge work?
Usually, yes. This is a temporary decline, so a retry a few days later often succeeds.
What to do about it
Retry a few days later, ideally just after common paydays (the 1st and 15th). Most recoveries for this code come from retries, but an email asking for a different card catches the rest.
What to tell your customer
Your payment didn't go through because the card had insufficient funds. You can try again later or update to a different card.