Is the return deadline the "shipping date" or the "arrival date"?

The "date you complete the shipping procedure" is the standard.

On the last day of your rental, that is, by 12:00 noon on the 4th day, please be sure to complete the shipping procedure for the item you are returning at a nearby convenience store or shipping carrier office.

The date you actually complete the shipping procedure at the store or office is what matters, so it is not a problem if the returned item arrives at our store the next day or later.

As long as you make it in time for that day's pickup, you don't need to worry about the arrival date at our store, so you can return your item with peace of mind. To ensure you meet the return deadline, please cooperate by completing the shipping procedure by noon on the last day.