A. Ensure Customer Number cis an External ID and that a custom field Invoice Number cis an External ID and Upsert invoice data nightly.

B. Use Salesforce Connect and external data objects to seamlessly import the invoice data into Salesforce without custom code.

C. Create a cross-reference table in the custom invoicing system with the Salesforce Account ID of each Customer and insert invoice data nightly,

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.