Question: When a TN extension by mail is pending and if I then choose to go to the border for the TN, do I need to tell USCIS that I want to withdraw the application?
Answer: You should notify USCIS to withdraw the petition. Generally, the border will not adjudicate a TN application when there is a TN petition pending with USCIS. You should keep a copy of the letter requesting the withdrawal and proof that USCIS received the letter (certified/return receipt). It may take several months before you get any response from USCIS regarding the withdrawal, so it’s important to keep evidence of the request if you plan to apply at the border before you receive a reply from USCIS.