The invoice (screenshot) looks like came from an unprofessionally designed site which might be a scam.
(reuploaded your screenshot to imgur to be displayed here)
The image looks no CSS design and it looks like he is dealing on the darknet or deepweb? I never saw this kind of payment request to any well-known payment gateway online.
He might be dealing on the Tor website that is why the payment page looks unprofessional or not well-designed payment page.
I'm thinking it's a error with site because ı've ordered something on here with escrow before.I know the site is reliable but ı can't tell the name of site .I'm waiting ı don't have anything to do .
They are asking it because they want to test or check the site if it is legit or scam. So without them, we can't give you a proper answer to why you are having this issue.
It is hard to give a detailed answer without enough details about the issue.