Incorrect future invoicing for an amended order product might be caused by the following scenarios:
A: Not contracting the Amendment Order: If the amendment order is not contracted, the changes made in the amendment order will not be reflected in the future invoices. This is because the contract is the legal agreement that specifies the terms of the sale, including the products, quantities, and prices. If the amendment order is not contracted, the original terms of the sale remain in effect, which may result in incorrect future invoicing1.
B: Having a subscription on the Contract be linked to an Order Product with one or more unpaid invoice lines:
If there are unpaid invoice lines linked to the order product, it may cause discrepancies in the future invoices. This is because the unpaid invoice lines may still be pending for payment, and if they are not resolved before the amendment order is contracted, the future invoices may not accurately reflect the changes made in the amendment order2.
C: Having a subscription on the Contract be linked to an Order Product with one or more invoice lines in draft:
If there are invoice lines in draft status linked to the order product, it may cause inaccuracies in the future invoices. This is because the draft invoice lines have not yet been finalized and may still be subject to changes. If these changes are not finalized before the amendment order is contracted, the future invoices may not accurately reflect the changes made in the amendment order2.
References: Salesforce Revenue Cloud documents12

The error 'attempt to de-reference a null object' typically occurs when there is an attempt to access a field or method on an object that has not been initialized or has been assigned a null value. In the context of Salesforce Revenue Cloud, when amending legacy contracts and subscriptions, thiserror can occur if required fields are missing or incorrectly populated on the legacy contract and subscription data. This could be due toincorrect data migration or manual data entry errors. It's important to ensure that all required fields are correctly populated when migrating legacy data to Salesforce CPQ.
References
Error:Can't renew the contract automatically because it ... - Salesforce Guidelines for Amending Contracts - Salesforce Legacy Data Upload for Salesforce CPQ

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 References:
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

Upon noticing unacceptable performance times in Salesforce CPQ, particularly when the primary quote syncs to the opportunity, and identifying the cause as being due to extensive process builders referencing the quote and opportunity alongside other heavy customizations, the recommended strategy is to extend customization using coding best practices to improve scalability. This involves reviewing and optimizing the existing customizations, possibly refactoring process builders into more efficient code-based solutions that are better optimized for performance and scalability. This approach helps in addressing the technical debt and ensures that the Revenue Cloud solution is built on a solid and performant foundation.

User acceptance testing (UAT) is a process of verifying that a software solution meets the business requirements and expectations of the end users. UAT is usually performedin a test environment by a representative group of users before the software is deployed to production. UAT is essential for ensuring the quality, usability, and functionality of the software, as well as the satisfaction and adoption of the users. To make UAT efficient, some of the planning strategies that should be taken are:
Define and agree on acceptance criteria with customer: Acceptance criteria are the specific and measurable conditions that the software must meet to be accepted by the customer. They are derived from the business requirements and user stories, and they serve as the basis for designing and executing the UAT test cases.
Defining and agreeing on the acceptance criteria with the customer before the UAT phase ensures that both parties have a clear and common understanding of what constitutes a successful outcome, and reduces the risk of ambiguity, confusion, or disagreement during the testing process. 12 Train UAT testers on the new functionality: UAT testers are the end users who will evaluate the software and provide feedback on its performance, usability, and suitability for their needs. They should be familiar with the businessprocesses and workflows that the software supports, as well as the new features and enhancements that are being tested. Training the UAT testers on the new functionality before the UAT phase helps them to prepare for the testing activities, understand theexpected behavior and results of the software, and identify and report any issues or defects that they encounter. Training also increases the confidence and competence of the UAT testers, and improves the quality and efficiency of the testing process. 34 Finalize test plans before the build phase completes: Test plans are the documents that outline the scope, objectives, approach, resources, schedule, and deliverables of the UAT phase. They specify the test cases, scenarios, data, tools, and methods that will be used to verify the acceptance criteria of the software.
Finalizing the test plans before the build phase completes ensures that the UAT phase is well-planned, organized, and ready to start as soon as the software is delivered to the test environment. It also allows for early detection and resolution of any gaps, risks, or dependencies that may affect the testing process. [5] [6] References:
Learn About User Acceptance Testing
User Acceptance Testing Strategies for Large Data Volume Scenarios
5x Salesforce Professional Revenue Cloud Tests 2023
A Guide to Salesforce User Testing (UAT)