Wow - thats below accepted tolerance so document it, contact CB1 Medical and return it to the pharmacy. Let us know how Cb1 handle it. When you have time let us know the product, exp, batch pls.
The way to prove it sadly is filming opening and weighing it but another thing is to weigh the unopened packet as this would prove the total weight before opening and then if it is underweight the total weight of bag and contents has been recorded prior to opening ,then you would put a complaint in to your clinic who would escalate it to the supplier
My understanding is they arrange a pick up but in my previous experience they said to destroy it if I wished as it would be replaced but I will find out for certain if you can or can't .