Can you state this in your contact?
Yes. Exercising the entitlement to additional payment after the service has been provided does not change the facts that the customer (1) was duly informed at the formation of the contract, and (2) willfully and knowingly accepted that clause. In terms akin to the Restatement (Second) of Contracts at § 154(a)-(b), the language "at our own discretion" supports the argument that the customer agreed to bear the risk of being charged the additional €850. The clearer and the more visible the clause, the likelier it would prevail over consumer protection laws.
However, other language in the contract might suggest that the supplier's discretion regarding the €850 cannot be completely arbitrary. That language could have the effect of constraining the extent of "but not limited to". For instance, the language "damages or other loses due to your usage of our service" might afford the customer a position similar to ejusdem generis, and thus prevent the supplier from applying the additional charge unless the surcharge can be reasonably premised on some adverse event. This is why the supplier might have to be more explicit if his intent is to establish a broader entitlement to arbitrariness.
