How do you use google play discount in pokemon go?

I've used these a couple times. To use it, you'll need to go to the store in Pokemon Go and select something that costs real money instead of Pokecoins. When it brings up methods to pay for your selection, you should be able to see the discount applied there. If it's not applied, tap on the payment methods and you should be able to see it in the list along with any credit cards you have linked and Google Play credits (if you have any). It will cover part of the cost, and the rest will be applied to whatever normal payment method you use.