Hi Mark,
I think one thing you may missing is an understanding of the purpose and operation of the mail merge process.
Mail merge is used to 'personalize' a preset document using information from the records of a database. Any content that will vary from copy to copy of the document must be imported from the database. Every item that is specific to the copy being sent to a specific individual must be included in the row of the table containing the record for that individual.
For invoices, unless you are sending an invoice for the same item(s) at the same price(s), you'll need to either include the items and prices in the database and merge them into the invoices, or use the merge document to create a document containing an empty invoice for each customer, containing only the customer identification information, and the company information (taken from the card marked "My Card" or "Me"), then fill in each invoice manually.
The Pages '09 template named "Invoice" might word with the second approach, depending what type of document the merge to file option produces. If it's a Pages document, the table, containing the invoice data, will likely be useable; if it's a pdf document, the table may be editable (or not), but will lose its ability to calculate the tax amounts and totals.
To produce an individual invoice from this template:
Open the template.
Drag the recipient's card from Contacts/Address Book and drop it on one of the fields in the first block of address text.
Enter the project information in the second block of text.
Check the Company block in the left margin for correctness.
Enter the Invoice items and quantities in the table.
Print (or Print to PDF) the document and mail it.
Regards,
Barry