do i need to return charging cord for macbook air

No. You can keep accessories and cables.

click here ➜ Apple Trade In - FAQ - Apple

Ronasara wrote:

However, if you do not return the power cord, you will have your trade-in amount reduced. This is noted in your instructions.

Incorrect. The cables and accessories are not required, and not including them will not reduce the trade-in value as clearly explained in the support document linked in my reply above.

Do I need to return accessories to get the full trade-in value?

* You don’t have to, but you can always send in your chargers, cables, cases, and watch straps and we will recycle them for you.

However, if you do not return the power cord, you will have your trade-in amount reduced. This is noted in your instructions.