Hi ed-carron,
Providing receipts is an important part of the sales process and I'd be happy to help you with this.
QuickBooks Online makes sending invoices easy by providing the option to email or print them out. If you already received a payment from your customer and want to issue the receipt, you just need to open the payment and select either print to print it out or save and send to email the receipt to the customer. A quick way to find the payment is either opening up the customers profile by going to Sales>Customers or by going to pull up your recent history by going to Sales>All Sales.
Hope this was able to help!