We don't use a standard "one size fits all" invoice format. We tailor our invoices to the type of event we are billing for.
For catering our invoices are itemized and reflect the contract the customers signed up front. The contract lists the menu items, quantities, amenities (decorations, etc) and prices of what they ordered. The invoice is just a "bill" for the contract amount with the same data listed out and totaled up. Itemizing may be a bit of a hassle but it protects both you and the customer. Detailing the bill avoids assumptions on both sides.
I used MS Word to create a master document with Data Form Fields and have it linked to an MS Spreadsheet. The spreadsheet has the contracted items and they auto-fill into the word document to create the "invoice". A new document is "created" from the master for each event and archived, once payment is received, for future reference.
We require 50% of the total contract paid upon signing of the contract. The contract states that this is non-refundable after one week before the event. If they call prior to one week before the event, to cancel, the contract states that 25% is non-refundable as a deposit. The reason for these non-refundable amounts is that we need to recoup the Cost of Goods and labor we will have expended in preparing for the event as well as possible loss of sales for that date (i.e. we may have turned away other sales for the same date). This is pretty standard in any service industry, especially the catering industry.
External Links for Invoice Templates & Apps: