There can be some cases in which you have introduced your T&T codes in your e-commerce backend and they are visible within the order in Channable, but they are not forwarded to the marketplace.
Whenever this happens, make sure you check your transporter's configuration in your order connection setup in Channable. If it's empty, or you are using transporters that are not included here, the T&T codes won't be propagated to the marketplace.
If you are making use of our Channable API for your order connections, revise that POST request you are sending to the corresponding endpoint is correct.