I'd like to help you record your sales, sarah17.
You use a Sales Receipt when your customer pays you right away. On the other hand, an invoice is used when your customer decides to pay you later. Only open invoices and unapplied credits will show in the Receive Payment page.
If you’d like to apply a credit memo to a sale, you’ll want to use an Invoice instead.
You can read through these articles between the difference of Sales Receipt and Invoices.
Let me know if you have other questions about receiving payments.