For both new license and license renewal payments, you will receive a payment confirmation email with the invoice attached as a pdf file. Consider this invoice your receipt.
At any time you can download an invoice by going to the My Invoices page on the My Intuiface website and clicking the "Download" link in the Invoice column of the row corresponding to the purchase you're interested in.
NOTE: The above procedure only applies to licenses initially purchased on or after July 2016 - i.e. for payments handled by the payment processor Stripe. If the original purchase date was earlier than July 2016, see this article about how to retrieve invoices for payments processed by 2Checkout.