When I click on "PAY NOW" why do I see an option called "THIS INVOICE IS ALREADY PAID" on the pop-up screen [Quickbooks]?

You may have invoices in Quickbooks which were fetched to Remitr but were paid subsequently outside Remitr, via cheque or EFT. If the invoice was marked as Paid in Quickbooks and you attempt to pay it again via Remitr, you will be reminded that this invoice has already been paid.