There are a variety of great desktop publishing software choices that make creating invoices simple and easy. Additionally, you can find some very affordable software programs that are created just for this purpose and can range anywhere from $10-$40 such as My Invoices, QuickBooks Invoice Manager, and Nova's Invoices and Estimates Pro just to name a few. You could even use standard word processing software such as Microsoft Office to create your invoice. Which ever program you use, make sure that you can save your invoice as either a .doc file, .pdf, or a graphics file so you can then send it to your clients as an email attachment. Alternatively, you could copy and paste the invoice directly into the email itself.
One of the benefits of this method is that you have total control over how the invoice looks and if you ever need to provider a hard copy of the invoice for any reason you can easily access it and print out a copy; otherwise this lets you take care of billing and your invoice needs with no need to waste paper or spend money on postage.