Log in to https://app.mypaymash.com/ and go to Sales. In the search box at the top right, enter the name of the customer. Immediately, only invoices issued to that customer will appear.
Alternatively, you can filter by invoice number or amount due.
In the app
Tap on "Select customer" and select the customer in the list on the left. On the right you will see the information about this customer and under the item "Last purchase" his invoices are listed (the last five). You can click on each invoice and reprint the receipt or cancel the invoice as needed.