In WHMCS, you can configure paid invoices to be “auto numbered” when they’re paid. This changes the invoice number they have in WHMCS.
For example, your invoice #20 gets paid, and it’s the fifth invoice you mark as paid. It’s now marked as Paid#5 (if this happens to you – start using that “Send reminder” button by the way)
But what if you’re given the old invoice number, and need to look it up? Well, then the Search function is there for you. If you enter the invoice number in the Search function, the invoice (with it’s old number) will appear as one of the results, as you can see in the image below.