Why can't I pay the rest by card?

CSFC always spends your balance first and charges the remainder to your card. Usually that is invisible. Occasionally the remainder is very small — and card networks will not process a charge below a floor.

What to do

Top your balance up so it covers the whole price, then join. A small credit pack is the quickest route, and the leftover stays on your balance for next time.

Retrying the same payment will not help — the amount is the problem, not the card.

Why the app tells you early

The join screen checks this before you confirm, so you find out on the payment summary rather than after being sent to checkout.