Equipment, set dressing, wardrobe and production materials that came into México as temporary imports must be returned to the U.S. in much the same way they entered the country. Each item was part of a shipment that was listed on a pro-forma shipping invoice and its counterpart -- a pedimento.
As each full or partial shipment leaves México, the pedimento it came in on must be canceled. When all is said and done, all the pedimentos your production generated must be accounted for. Un-canceled pedimentos are subject to fines of 400% of the declared value of the shipment. The best of all possibilities would be to return each shipment (as listed on the original proforma) in the same (or as close to the same) configuration as it was sent down, thus allowing complete pedimentos to be canceled. As you are ready to return individual pieces of equipment that came as part of larger shipments -- you will be canceling just that portion of the pedimento it came into México on. If you are returning several pieces of equipment, set dressing, props, etc. that all came in at different times as part of different shipments -- each item must be matched up to its original pro-forma and pedimento.
Since your custom broker must go through the same procedures to cancel an individual item on a pedimento as they do to cancel an entire pedimento -- returning an item or two at a time could become more costly than waiting to return an entire shipment at once. Unless you have a flat deal on both sides of the border for making returns, weigh the costs of canceling individual items with the value of the item(s) you are returning or the rental you are paying on those items.
Your custom broker will generate a return pedimento for each returning shipment. It may contain items from a single incoming pedimento or from several. Once the return pedimento is issued, the load cannot be altered in any way. You cannot decide to add something at the last minute, take something out of the truck or switch one piece of equipment for another. If the paperwork does not match the load exactly (down to the correct serial numbers), the truck could be held-up at the border indefinitely, and the incorrect pedimentos will have been canceled.