1. Find the transaction. Go to Sales, then All Sales. Filter by type if you need to, Sales Receipts for a sales receipt, Invoices for a paid invoice, and open the one you want to send.
2. For a sales receipt, open it and select Save and send. This emails a PDF copy to whatever address is on the customer's profile. You can edit the subject and message before it goes out.
3. No email on file, or sending it another way? Use Print or Preview instead. Choose Print or Preview, then download the PDF. From there you can attach it to a text, a different email account, or print a physical copy.
4. To send a payment receipt after recording an invoice payment, start from Receive Payment. Open the paid invoice, select Receive payment, enter the details, and save.
5. On the confirmation screen, choose the send option. After saving, QuickBooks offers to email the customer a receipt confirming the payment went through. This is a separate document from the invoice itself, it's just proof of payment.
6. Resending something already sent. Open the same transaction again and repeat the Save and send step; QuickBooks doesn't track whether something was already emailed, so resending is exactly the same process as sending it the first time.