Invoices for Salesforce functions in the exact same way in all the supported editions of Salesforce with the exception that Professional Edition users need to mantain the customization of the Invoice and Invoice Line Item objects manually.

The manual customization of the Invoice and Invoice Line Item objects is required every time that:

• A new invoice template is created or an existing template is edited, and the new or edited template includes new merge fields. For more information see Invoice Templates
• A new billing rule is added to a billing scenario, and the billing rule field is different than all the fields of the existing billing rules. For more information see Billing Scenarios

So basically the manual costumization of the Invoice and Invoice Line Item objects is only necessary during the setup stage when you create your invoice templates and billing scenarios. This is a one off activity unless you need to make changes.

Although you need to customize the Invoice and Invoice Line Item objects manually, you do not need to be concerned about when this customization is required. The application keeps track of your new invoice templates and billing rules, or changes in them, and signals you when it is necessary to customize the Invoice and Invoice Line Item objects. This is best described with an example. Consider the first time you setup the application. The steps you would follow would be:

1. Create a new invoice template as described in Invoice Templates
2. Create a new billing scenario and add to it the billing rules as described in Billing Scenarios
3. Prepare you sales for billing as described in Preparing your Sales for Billing

At this point the application is setup and you are ready to process your sales' invoices. It is at this point, right after you created your invoice templates and billing scenarios, that the application signals you that you need to customize the Invoice and Invoice Line Item objects. The way this happens is message screen pops up in the following two events:

• You try to process invoices in single or bulk mode as described in Creating and Processing Invoices
• You try to edit an existing template or create a new one.

An example of the message screen that signals you the need to customize the Invoice and Invoice Line Item objects is shown below.

At this point you need to customize the Invoice and Invoice Line Item objects according to the information in the message screen table before proceeding. In order to customize the Invoice object follow these steps:

1. Go to You Name | Setup | App Setup | Create | Objects.
2. Click on Invoice.
3. In the Custom Fields & Relationships related list click the New button to create a new custom field.
4. In the New Custom Field page select the Data Type of one of the entries in the message screen table as in the example above, with Object name equals Invoice2__c, and click Next.
5. Enter the text for the Field Label according to how you want the label appear, and enter the Field Name exactly as it appears in the message screen table entry. Enter the additional required values depending on the field type and click Next.
6. Check the Add Field checkbox if not checked already so that the new custom field is included in the Invoice object layout and click Save.
7. Repeat steps 4 to 6 for every entry in the message screen with Object name equals Invoice2__c.

To customize the Invoice Line Item object follow the same steps as with the Invoice object above, except that you use the message screen table entries with Object name equals Line_Item_2__c