If you're invoicing from the appointment book, use the drop-down arrow to the right of the Save button and select the Save & Email option. This will email the receipt to the email listed in the Bill To section:

Similarly, if you're creating a new invoice from the patient or third-party payer file, you can issue a receipt using the same drop-down menu option:

If you need to re-issue a receipt for a fully paid invoice, simply click the 'email' button at the top of the invoice page:

PracSuite includes a default receipt template, though you also have the option to create your own Email Templates. Invoice email template preferences can be set per Business in Settings > Businesses > Businesses as shown below: