Requesting a deviation from the customer {I assume} means that you aren't going to follow a mutually agreed requirement.
Customer relationship manager (sales, engineering, other} would be my first pick...together with the person who can best make the case for what, why, how long and potential impacts.
If you aren't going against mutually agreed items, why involve the customer at all?
At the end of the day, it should be done by the person who can succeed in getting approval without irking the customer...regardless of position or title.