[mothered]

from what I understand, it can take 24 hours for 5 pounds of dry ice to sublimate?

Under normal atmospheric conditions.

so given that I maybe drop the box off at my post office in the morning knowing that it won't be collected until the evening and then at the very earliest it would be delivered the following morning back to the warehouse, this in my mind leaves enough time for the dry ice to evaporate.

How certain are you of the above storage & transportation?

Have you researched their procedures?

also am I right in saying that the box only needs to weigh the correct amount at the time of returning it to the post office right?

That's provided It's not taken to a depot awaiting allocation to Its destination.