Upon noticing an error on a posted invoice, the recommended corrective action is to use the "Cancel and Rebill" feature. This process allows an admin to roll back the invoice to its state before the most recent billing cycle, enabling the correction of errors on the invoice record. This functionality is applicable to both draft and posted invoices, providing a way to address issues such as missing invoice lines or incorrect billing details. After canceling and rebilling, the admin can correct the order as needed and proceed to create and post a new invoice, ensuring the accuracy of billing records.

The correct sequence of evaluation events for a price rule, quote calculator plugin (QCP), and CPQ package pricing engine is as follows:
Internal Initialization: This is the first step where the system prepares for the calculation process.
On Initialization: At this stage, any price rules that are set to trigger 'On Initialization' are run.
Calculate Formulas: The system evaluates formula fields.
Before Calculate: Any price rules that are set to trigger 'Before Calculate' are run.
Calculate Quantities: The system calculates quantities, for example, bundle components.
On Calculate: Any price rules that are set to trigger 'On Calculate' are run.
Price Waterfall Calculation: The system calculates out-of-the-box pricing tools such as block pricing, discount schedules, etc.
After Calculate: Any price rules that are set to trigger 'After Calculate' are run1.
This sequence ensures that all calculations and price rules are applied in the correct order, providing accurate pricing information1. Reference:
https://help.salesforce.com/s/articleView?id=sf.cpq_price_rule_considerations.htm&language=en_US&type=5
https://help.salesforce.com/s/articleView?id=000388745&language=en_US&type=1

To implement the user story of rewarding loyal customers with bulk discounts based on previously purchased quantities, the design of the solution should include the following components:
Contracts, Subscriptions and Assets should be populated with historical data. This is necessary to track the customer's purchase history and determine the appropriate discount tier for each product or service. Contracts, Subscriptions and Assets are the core objects of Revenue Cloud that store the information about the customer's agreements, recurring charges, and entitlements. By migrating the historical data from the legacy system to these objects, the customer can leverage the Revenue Cloud features such as renewal management, usage-based pricing, and revenue recognition. 123 Discount schedules with Cross Orders checked. This is the feature that enables the bulk discounts based on previously purchased quantities. A discount schedule is a set of discount tiers that apply to a product or a product option based on the quantity or amount ordered. By checking the Cross Orders option, the discount schedule will consider the quantities from all the orders associated with the same account, contract, or subscription. This way, the customer can reward their loyal customers with lower prices for higher volumes. 45 Reference:
1: Revenue Management Platform & CPQ Solution - Salesforce.com US
2: Salesforce Introduces Revenue Cloud to Help Businesses Accelerate Revenue Growth Across Any Channel - Salesforce
3: Give Discounts for Long SubscriptionsUnit | Salesforce Trailhead
4: Discount Schedules - Salesforce
5: Salesforce Revenue Cloud Trailmix - Trailhead

The three common CPQ design mistakes to avoid while implementing CPQ for your customer are:
A . Using price book entries to handle attribute based variationsinstead of lookup price rules: Price book entries are not designed to handle attribute-based variations. Instead, lookup price rules should be used. These rules allow you to adjust pricing based on specific product attributes, providing more flexibility and accuracy1.
C . Creating process builders and flows to pass data between fields instead of using twin field mapping: Process builders and flows can be complex and difficult to maintain. Instead, twin field mapping should be used. This feature allows you to map fields between objects, ensuring data consistency and reducing the need for custom code1.
D . Writing customizations for product selection or validation instead of using option constraints, product rules, and bundles: Customizations can be time-consuming and costly to implement and maintain. Instead, Salesforce CPQ provides built-in features like option constraints, product rules, and bundles that can handle product selection and validation. These features are designed to enforce business rules and ensure that only valid product combinations are selected1.
Reference:
https://inspireplanner.com/blog/5-common-salesforce-cpq-implementation-challenges-and-how-to-avoid-them/