Legal for trade weighing using a PLC
I think you're going to need to seal the signal pathway from the weighing platform to the printer such any physical tampering will be evident, this means some sort of cover over the moving parts and the wiring

And also restrict operator customisiation such that they cannot print false labels. If necessary move the printing code out of the PLC that's controlling the line, into a controller that is only responsible for putting marks on paper 
 and can be locked down.  (possibly with an auxiliar output only signal path to the controlling PLC so the controller can see the weight, and some sort in input to command it to weigh and print.