How can I pay with my Apple ID funds

Seems your Apple ID funds are mostly used first,

whenever making valid Apple purchases from Apple using your Apple ID.

Re: "How can I pay with my Apple ID funds"

May depend on what you want to purchase.

Seems you just purchase valid items from Apple, while logged in with your Apple ID on your device.

Sign in with your Apple ID - Apple Support

However, for some purchases Apple requires a valid credit card listed in your Apple ID account,

even if you are paying with your Apple ID balance. 

How apps, content and subscriptions from Apple are billed – Apple Support

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What you can buy with your Apple Gift Card or Apple Account balance – Apple Support

Find your country or region to see what's available to you from Apple..

Availability of Apple Media Services – Apple Support

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More info:

How to: Check your Apple Account balance – Apple Support

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For support from Apple, if needed:

Apple ID - Official Apple Support

Subscriptions and Billing - Official Apple Support

All the best :-)