I'll just add that unless you're planning on generating only a small number of invoices, using a spreadsheet for the task gets old pretty quickly. I'm a happy user of QuickSale - an invoicing and accounts app in the App Store that's well worth a look; they have occasional free trial offers, so worth checking out, though I think the entry-level paid version is only $10 or so. Although I only generate few invoices a month, I happily paid for the Pro version, and pretty much run my business on it. Recommended.
HTH Pete