if you are looking to save a new credit card on file without capturing a payment, view this help article. 

By saving credit cards, you increase the level of payment flexibility. Your customers don't have to enter their credit card information over and over while you can automate payments.

By default, the payment section of an invoice lets the customer select if they want their credit card saved.

If you want always want to save the card on file, you can activate the option "Save card on file for all payments" on the payment section of your invoice template. To do so, 

Screenshot of the "Edit" option from the payment section of an invoice template
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Screenshot of the "Edit" interface of the payment section of an invoice template
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A customer profile showing a saved credit card.