Explanation:

To determine the optimal way for Universal Containers (UC) to integrate invoice data into Salesforce so Sales Reps can see when customers pay their bills on time, we need a solution that efficiently links invoices from an external system to the corresponding Salesforce Accounts, given the unique Customer__Number__c field and the Invoice__c object with a Lookup to Account. The solution should minimize custom code, ensure data integrity, and support nightly data imports. Let’s evaluate the options based on Salesforce integration best practices.

✔ UC uses Customer__Number__c as a unique identifier across all systems, making it a key for matching external invoice data to Salesforce Accounts.
✔ The Invoice__c object has a Lookup to Account, requiring the Salesforce Account ID to establish the relationship.
✔ The integration should handle large datasets (implied by "nightly" imports) and avoid performance issues like excessive SOQL queries.

A. Ensure Customer Number__c is an External ID and that a custom field Invoice Number__c is an External ID and Upsert invoice data nightly.
This approach designates Customer__Number__c on the Account object and a new Invoice_Number__c on the Invoice__c object as External IDs. An External ID enables upsert operations to match and update records using these unique fields instead of Salesforce IDs. The nightly upsert can use Customer__Number__c to link invoices to Accounts and Invoice_Number__c to identify or create Invoice__c records. This minimizes SOQL queries, supports bulk operations, and ensures data integrity with minimal custom code (e.g., via Data Loader or an ETL tool), making it highly efficient and optimal.

B. Use Salesforce Connect and external data objects to seamlessly import the invoice data into Salesforce without custom code.
Salesforce Connect with external objects allows real-time access to external data without importing it into Salesforce, using OData or other protocols. While this avoids custom code and nightly imports, it requires the external system to expose invoice data via a supported adapter and does not store the data locally for Sales Reps to view payment status historically. This approach is less suitable for tracking payment timeliness, which implies a need for persisted data, making it suboptimal.

C. Create a cross-reference table in the custom invoicing system with the Salesforce Account ID of each Customer and insert invoice data nightly.
This solution involves maintaining a cross-reference table in the external system to map Customer__Number__c to Salesforce Account IDs, then using this to insert Invoice__c records nightly. This requires ongoing synchronization of Account IDs in the external system, adding complexity and potential for data mismatch if Accounts are updated in Salesforce. It also relies on custom integration logic to perform the insert, which is less efficient than leveraging Salesforce’s upsert capability, making this less optimal.

D. Query the Account Object upon each call to insert invoice data to fetch the Salesforce ID corresponding to the Customer Number on the invoice.
This approach queries the Account object for each invoice insert to match Customer__Number__c to the Account ID, then sets the Invoice__c Lookup. For a nightly batch, this results in numerous SOQL queries (one per invoice), risking governor limits (100 SOQL queries per transaction) and poor performance with large datasets. This is inefficient compared to using External IDs, making it the least optimal solution.

Correct Answer: A. Ensure Customer Number__c is an External ID and that a custom field Invoice Number__c is an External ID and Upsert invoice data nightly.

Reason:
✔ Designating Customer__Number__c as an External ID on Account and adding Invoice_Number__c as an External ID on Invoice__c allows upsert operations to match records using these unique identifiers, eliminating the need for SOQL queries.
✔ The nightly upsert can process thousands of invoices efficiently, linking them to Accounts via Customer__Number__c and creating or updating Invoice__c records with Invoice_Number__c, ensuring Sales Reps see payment status.
✔ This approach leverages Salesforce’s built-in upsert functionality (e.g., via Data Loader, MuleSoft, or Apex), minimizing custom code and optimizing performance for large-scale integration.

Implementation Notes:
✔ Set Customer__Number__c as an External ID field on the Account object (via Setup > Object Manager > Account > Fields & Relationships).
✔ Add Invoice_Number__c as a custom field on Invoice__c, mark it as an External ID, and ensure it’s unique.
✔ Use an ETL tool or Apex batch job with Database.upsert() to process the nightly CSV or API feed, mapping Customer__Number__c to Account and Invoice_Number__c to Invoice__c.
✔ Include payment date fields (e.g., Payment_Date__c) in the invoice data to track timeliness.
✔ Test with a sample dataset in a sandbox to verify matching and upsert behavior.

Reference: Salesforce Integration Guide - External ID Fields, Salesforce Data Loader Guide - Upsert Operations.