Product or service details: Specify Description, Quantity, Rate, Amount and Subtotal. The more details you include on the invoice, the better. This way, your client will know exactly what they are paying for upon receiving the invoice. Make sure that your client can understand every item, so that as soon as they receive the invoice, they can pay you instead of asking questions and delaying the process.
Both of these services can be used for free or upgraded for a small fee, and include lots of other cool features, like scanning receipts for work completed in the field.