Below the total you can include any additional information, such as a thank you or a reminder. - When you work out the contract with each client (and yes, you should have a contract in place!), you'll list out the terms of how often you plan to invoice and when you should be paid. For instance, you can agree to bill on the first of each month, or biweekly so you get paid every other Friday.
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.