Do I remove my SIM card from my iphone before sending it for a trade-in?

Remove the SIM so you can use it in the new phone.

It’s your choice. Do it if you wish. It’s not required.

If you choose to use your old SIM in your new iPhone, remove it. Or you may purchase your new iPhone with a new SIM and use that one.

Sorry, I meant to address this to Jlklein86.

Your old SIM is already set with your phone number. If you send it in, somebody could use it in another phone and run up charges on your phone bill until you have your carrier disable it and assign your phone number to a new SIM.