How do I make sure I pay Apple Card's monthly payment only?

You can schedule to make just the Minimum payment by following this Support Article:

How to make Apple Card payments - Apple Support

1. On your iPhone, open the Wallet app and tap Apple Card.
2. Tap the More button, then tap Scheduled Payments.
3. Tap Set Up, then tap Continue.
4. To choose the amount that you want to pay, tap Monthly Balance, Minimum Payment Due, or Other Amount.

• • If you chose Monthly Balance or Minimum Payment Due, select when you want your payment to be made. Then tap Next.
  • If you chose Other Amount, select your payment amount, when you want the payment to repeat, and when you want the first payment to be made. Then tap Next.

1. Confirm with Face ID, Touch ID, or passcode. Then tap Done.