Hi there JAL88,
Thanks for reaching out about sending receipts to customers. QuickBooks Online is a great tool able to help you keep track of sales and expenses so you know the profit of your company. I can provide some information about sending transactions to customers.
The only way to send transactions to customers, like payment receipts or invoices, would be to click the Save and Send option when trying to save the transaction. Creating a sales receipt on top of the invoice and payment would actually be doubling the income in your books, so you would need to either click save and send next to the payment when creating it, or go into the invoice after the fact and send it to them after having applied the payment to show there is no balance left on the invoice.
If you have any other questions, feel free to reach out here.