Opportunity in Salesforce can easily be integrated with AccountingBook to create Invoices or Sales Order.

Usage

Anytime an opportunity is "Closed" you would want to generate a transaction (Sales Order or Invoice). You can have multple scenarios that you can model with Salesforce and AccountingBook

• Services: Quotes -> Opportunity -> Invoice
• Manfacturing: Quotes -> Opportunity -> Sales Order -> Invoice
• Retain: Opportunity -> Sales Receipt or Combining daily closed Opportunities -> Journal Entries
• Subscription: Recurring Opportunity -> Recurrning Invoice Template -> Invoices(s)

Installing Standard Out-of-the-Box Integration

Out of the box provides an (unmanaged) i.e. open source code for integrating Opportunity to Sales Order, Invoice or Sales Receipt. 

Pre-requistes for this package are:

1. AccountingBook should be installed
2. Salesforce Enterprise Edition (with access to Opportunity & Products)

Install Opportunity to Receivable package from https://login.salesforce.com/packaging/installPackage.apexp?p0=04to0000000MZUv