All in-game purchases made in June's Journey are handled by Google Play.
If you experience an issue, such as a declined payment or invalid account, please visit the Google Play help page of troubleshooting steps. You can find it here.

Available payment methods may vary by country. Listed below are the most common methods:
  • Credit or debit card
  • Google Wallet balance (only available in the US and UK)
  • Direct carrier billing
  • Google Play balance or Google Play gift card (click here for more information)
  • PayPal
Read the full article on accepted payment methods on Google Play Help page.