I think it being a real invoice would be the easiest answer. The scammers have obtained a legit business account at PayPal and are issuing baseless invoices. I

I think it being a real invoice would be the easiest answer. The scammers have obtained a legit business account at PayPal and are issuing baseless invoices. I don't know if they'd receive the money if you paid - maybe. But they want you to call them and install the backdoor.