Do you want the ability to automatically send customers their invoice prior to it being due? You can with Unit Trac. 

Step 1 - Go to Business > Settings > Notifications

Step 2 -  Select 'Enable automatic invoicing' under the Upcoming Invoice Notifications section to automatically send an invoice by email to your customers before they are due;

Step 3 -  Select the number of days ahead of the due date that you'd like to have your invoices emailed out.

Step 4 -  Select your customers to receive the invoices by clicking   and then selecting them from the listing of available customers with an email address. 

For more info on customizing invoices see; add your logo help article, or add custom notes.