They can ask for it, since no one is forced to do business with any particular company: if you don't like their terms, you don't have to hire them. But they have to put those payment terms into the contract you signed: once you sign the contract, you and they both are held to the terms, including payment terms, in the contract, and they cannot change them. You only have to pay when and as the contract indicates. So check the contract to see what it says about payment.