How do I select a specific payment option when downloading an app from the App Store?

When downloading an app from the App Store you don't get payment method choices. It automatically uses the payment methods on your account in the order they appear in.

If the App has subscriptions and you allow the free trial to renew, it will again use the payment methods on your account in whatever order they appear in.

To change the order, go to Settings ➜ Account ➜ Payment & Shipping and you will see your payment methods listed. You can re-order them there by tapping on "Edit" and dragging them up or down on the list. The top most payment is tried first to pay for apps and subscriptions. Your Account Balance is always the first thing it tries to charge.

For subscriptions that have options for how often they charge, weekly, monthly or yearly you need to choose one based on what you want to pay and how often you want to pay. The subscription continues until you cancel it, and will renew on that time scale ,Weekly, monthly or yearly, and charge you automatically every time.

It will still use whatever payment methods you have added to your Apple Account in the order they appear in. Trying each payment method listed until it finds one it can charge. If it is unable to charge any payment method, then the charge will remain pending until it can be successfully charged and your account will then be in bad standing preventing you form downloading any apps free or paid and updating existing apps until the charge is completed successfully.

If you change the order, and then all purchases and subscriptions will use the new order starting with the top most payment method. There is no option to assign a specific payment method to a specific purchase or subscription.

click here ➜ If you need to change or update your Apple Account payment method - Apple Support

From this support article --> How apps, content, and subscriptions from Apple are billed - Apple Support

"Which payment method is charged?

When you buy items or pay for subscriptions, Apple automatically attempts to charge your payment methods in this order:

1. Apple attempts to use your Apple ID balance to cover the full amount.

2. If you don't have Apple ID balance, you don't have enough to cover the full amount, or an item cannot be paid for with Apple ID balance, Apple attempts to charge your primary payment method (typically a credit or debit card, but varies according to country). The primary payment method is at the top of the list in Settings on your iPhone. You can control which payment method is at the top of the list. [See: Add a payment method to your Apple Account - Apple Support ]

3. If there's a problem with the primary payment method, or if the primary payment method can't be charged for the full remaining amount, Apple attempts to charge your other payment methods in order from top to bottom. If none of them can be charged, Apple may try again."

You cannot designate a specific card for a specific subscription. You would have to change the card order by moving the preferred payment to the top of the list prior to the item being charged, and then change it back again.

Here's how to change the payment method order for your Apple account after Apple account balance (which will be charged first in all cases unless the item you are buying cannot be charged to account balance). For the first four steps, refer to the first image in this Support article Add a payment method to your Apple Account - Apple Support where it says to tap Edit to reorder or remove a payment method.

1. Tap on the Settings app.

2. Tap on the Apple ID section at the top of the page (your name).

3. Tap Payment & Shipping (on old versions this is iTunes & App Store).

4. Tap Edit.

5. Tap and hold on your preferred Card and drag it up or down the list.

6. Tap Done.