Currency conversion fees.
The fees may be the least of your problems.
There may be serious limits on withdrawing the money in the U. S. It could be restricted and take months or more. Your transfer could be seen a money laundering and your account limited for 180 days. You may run afoul of internation money transfer regulations.
Assuming you are sending from your Mexican account while in Mexico to your U. S. account for use AFTER you return to the U. S., it might work technically. On the other hand, if you attempt to access an account while not in the country, you could have serious issues with security risks.
A much better option would be wire transfer bank to bank. Banks will make sure it is OK.
