Does the invoice say 1 month? The last time I used the public cloud GPU instance there was a very explicit setting to bill hourly or to get a discount by getting a month in advance.
It just says 1200€ in credit, but it's exactly the cost oof that instance if you have it hourly for a month. Ordering the month would have been lower than that.
I wonder if there are shenanigans involving whether you're paying with a credit card with credit and a bank account that they might not be able to preauth. Has OVH phone support been of any use?