Errors appear in a red banner for a few seconds:

• "Error: Quantity!" - the quantity is above the add-on's maximum; lower it.
• "Invalid Addon" / "_Invalid Addon" - the add-on or tier no longer exists; reload the page.
• "Unable to process the charge" - the item has no price or the charge link could not be created; contact support.
• A card message such as Your card was declined - the charge to your default card failed; update the card on Settings > Billing and try again.
• "Try Again!" / "Ups! Something is Wrong!" - the billing service did not respond; wait and retry.

If the charge went through but the add-on did not visibly activate, contact support rather than buying again - the charge is still on your billing account.