6 files declare their use of CheckoutInterface in Commerce PayPal 8

CheckoutController.php in src/Controller/CheckoutController.php

CheckoutPaymentProcess.php in src/Plugin/Commerce/CheckoutPane/CheckoutPaymentProcess.php

commerce_paypal.module in ./commerce_paypal.module

Implements PayPal payment services for use with Drupal Commerce.

CustomCardFieldsBuilder.php in src/CustomCardFieldsBuilder.php

PaymentMethodAddForm.php in src/PluginForm/Checkout/PaymentMethodAddForm.php

SmartPaymentButtonsBuilder.php in src/SmartPaymentButtonsBuilder.php