When someone has paid you using PayPal (e.g. with the Registration Payment System) you will find details of the payment transaction by logging onto your account at PayPal. The transaction details include an invoice number. If ever you need to manually accept or upgrade a membership registration you need this invoice number. The following screen shots show where you can find the invoice number:

Step 1: Log on to PayPal:

Step 2: Click on the history tab:

Step 3: Find the payment you're interested in (you may have to browse for it) and click the 'details' link:

Step 4: Either of the following pages will now appear. It is on these pages that you will find the invoice number:

Or
For subscription payments you may get a page like the one one below in which case click on the circled link:

this will bring up the details of the subscription including the invoice number: