Any order adjustments (before printing): should require both parties approval.
Please, allow clients during the 'review stage' to change materials and layer height if required.
Should there be changes in the terms of service from either the client or hub provider, both parties need to approve of the changes before proceeding to the next stage.
As it currently stands, I've had dealings with 3D hub providers who have thrown on extra costs above the advertised, then proceeded with the payment stage. Forcing me to either pay or cancel the order entirely. Seems backward negotiating.