Explanation
To reflect the price values stored in an external ERP during the checkout flow, the requirement can be satisfied by overriding the computePricingCart method in ccrz.cc_api_CartExtension and making the callout in this method. This method is responsible for computing the pricing for the cart and its line items. By overriding this method, the user can make a callout to the external ERP and update the pricing information accordingly.
Salesforce References: B2B Commerce and D2C Commerce Developer Guide, Logic Classes, ccLogicCart Class