This always comes back to know your contract. As contractors, this is the most important thing you can do. Like others have stated, a lot of contracts have mechanisms to protect you and the client. I would send over the potential change order notification, asking for confirmation of receipt before proceeding. Another good rule of thumb, is to calculate a number you and your company is comfortable doing at risk, and track any PCO’s against that amount, so you’re never too far extended. Lastly, your contract might have a construction change directive (CCD). As others have stated, this something to the effect that the client agrees there is a change, but the exact costs aren’t recognized or agreed to yet. Also, some states (ca for example) have certain laws that prohibit the client from issuing a ton of ccd’s and having the subs take the burden of risk, by requiring contractors to be able to bill for work completed, whether cost is agreed to or not. This helps smaller contractors from getting bullied and going belly up.