I had the same yesterday, today I got a mail from GBFinance attached was the Black Horse detail including the deposit amount, which showed £3.5k more than I agreed. I assumed this was plug-in grant related, called my SC to check and they confirmed it was, and that I didn't need to pay it
Also asked about the order deposit (£750 in my case); I've had a credit note for this in my Tesla account but no actual refund to my card - again the SC confirmed that I could reduce my payment by this amount, as they offset it.... So to summarise - Invoice shows £9500 deposit to pay
less the £3500 plug-in grant and the £750 order deposit - so £5250 to pay
Edit: Or put a little more simply - what Mr H said