In the second expression "gets" can be used. However "gets" can't be used in the third example.
There is a problem with the use of "when" in the second example. It suggests you strongly believe that the order will be cancelled; this seems unlikely. Use "if" instead.
The last sentence is wrong. "get ready" is a phrase. It means "prepare oneself", and doesn't have an object. You can't say "I get ready the order". You can say "I get ready for work". It would be possible to say "I get the order ready", so "You can pay when the order has been gotten ready by us." is grammatically correct. However, it is a horrid, unnecessary and ugly use of the get passive.
A natural way of expressing this is:
You don't need to pay until the order is ready.