MySQL trigger set values for NEW row and update another in the same table

Question

I have a table that I keep track of fees for a specific item. These fees can change over time so I have two columns (startDate, endDate) with the current set of fees always

Answer 1

From the MySQL Docs:

Within a stored function or trigger, it is not permitted to modify a table that is already being used (for reading or writing) by the statement that invoked the function or trigger.

You will need to find some other way of doing what you're trying to do.