Is it true to say “you can pay when the order gets ready”? Can we use “get + ready” for the things that will happen in future??
Everyone would know what you meant if you said "you can pay when the order gets ready," but it is a less accurate way of saying what you want to say. Using "gets" implies action. Actually, it implies that the order is taking action itself. It is making itself ready on its own. Obviously, that is not true. Someone is making or preparing the order. A better way to say this would be "you can pay when the order is ready." This works because "is" is a state of being, rather than an implied action. Alternatively, if you are talking about a person being ready for something, then it is perfectly fine to say "when he/she gets ready" because the person is performing the action of getting ready themselves, unlike an object. I hope that makes sense.
We would normally say "when the order is ready" to express that particular future eventuality.
We normally use get ready to express a person's preparedness for an activity, and it is important to note that it is a present action, something undertaken now, that prepares for the future necessity.
The objection that orders do not get themselves ready could be answered by rewriting the sentence this way:
You can pay when _____ gets your order ready.
Fill in the blank with a description of the person or persons you prepare the kind of order you are waiting for.
There is a second problem with this sentence, however, which is that the act of getting an order ready takes some time. It is usually a process with a beginning and an end. Do you pay when the process starts, or when it ends?
If the answer to that question is that you can pay at the beginning of the process, at the end, or at any time in between, one would more likely say,
You can pay while _____ gets your order ready.
One could imagine someone in a shop or restaurant saying this as a way to save some of the customer's time: by the time the order has been completely prepared they will already have paid and can leave immediately.
On the other hand, if the answer is that you pay at the end of the process (which is the most likely interpretation of the word "when" in this context), then the idiomatic phrase is "when your order is ready," as explained in the other answers. This is unambiguous enough, since the order is ready when the process has been completed but is not ready before then.