You defiantly also need to air tag those trailers for backup. The batteries can last up to 2 years before needing charge. You can name each airbag to show up on your phone as trailer 1, trailer 2 or whatever you name it. Put two on each trailer as redundancy.
If you have a tablet you might look into a digital contract form that the customer can sign on the tablet, and then you could email them a receipt. That should also let you do card based payments through Square or something similar.
