Hi, this is covered in the installment agreement when you purchased the items on an installment plan. Here is an Apple Support article that covers various aspects of the installment plan and the agreement,
https://support.apple.com/HT210644
Pay your monthly installment
Your minimum payment for Apple Card includes your Apple Card Monthly Installment. When you pay your minimum payment each month, you automatically pay the monthly installment for each of your installment plans.
Pay extra
To pay extra on your Apple Card Monthly Installments, you need to first pay your entire Apple Card balance. Then, to make an additional payment towards your installment balance, choose Pay Early. If you have multiple installments, your additional payment is applied to the outstanding balance of your oldest installment plan.
To Pay Early:
Open the Wallet app and tap Apple Card.
Tap the more button , then tap Monthly Installments. If you have multiple monthly installments, you see the balance for all of your monthly installments.
Tap Pay Early, then tap Continue.
Choose an amount to pay, then tap Pay Now or Pay later and follow the instructions on your screen.
If you don't pay a monthly installment, you aren't charged a fee or interest. To keep your account in good standing, pay your monthly installments on time as part of your Apple Card minimum payment due every month.
Because both items were purchased at the same time, you might have to pay both off. If your phone is older, than any additional payments are credited on that item/installment. I hope this helps.
