The PlayStation Store allows you to purchase gift cards, but because it's an entirely online store, you have to activate the cards with a code first.
This can easily be done from your PlayStation 4 if it has a connection to the internet.
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How to redeem a gift card code on your PS4
1. Log into your account on your PS4 and access the PlayStation Store. This should be a registered account with internet access.
2. From the PlayStation 4's Store menu, scroll down to the second-to-last option, "Redeem Codes," and select it.
3. Your voucher should have come with a 12-digit code that you can now enter into this box. If the code was bought as a digital purchase, like through Amazon, you might only have the code in an email. An example code will look something like: "94GP-QTNL-KKNN."
Once the code has been confirmed and checked, the value assigned to the code will be added as credit to your PlayStation Store wallet, ready to be spent.