Card processors refuse charges below a small minimum amount. An invoice for a few cents — the kind left over after a prorated upgrade or a partial credit — therefore cannot be paid by card, and the payment page will reject it.
This is a limit of the card network, not something we set, so retrying or using a different card will not help.
What to do: open a ticket and mention the invoice number. We clear balances like this from our side. Do not leave it sitting unpaid — a tiny outstanding invoice can still block other actions on your account.
Adding account credit has its own minimum, so topping up is not a way around a few-cent invoice either.
If you need help with this, please submit a ticket.