I feel robbed

Since I have been selling Spices I have been the only person to handle the Spices when they are delivered to me here in Portugal. I check the weight of every single pack before they are labelled, each spice or herb is in it's own sealed bag. After I place an order I then print the labels ready for when my next order is delivered. Each Spice or Herb has it's own label, it's really easy and and after the last couple of years there has been no mistakes Untill.

At the market at Miranda do CORVO I had a customer who after picking four items that he wanted then told me that I sold him a wrong item at the last market. On the pack it said White Cumin seeds 100g a very popular item I sell a lot of. 

To my surprise when I looked at what he gave me I could see that inside of the bag was not white Cumin seeds but in fact black Mustard seeds but also something else that looks like Nigella black onion seeds. The 100g bag when weighted had instead of 100g it had in fact 156g. Why someone would take Cumin seeds out of a plastic bag and put other Spices in it's place I don't know. Certainly I don't know why he did what he did as the cost of what he put in the bag cost more that what was originally in the bag. Each and every bag is heat sealed but this nag that was returned had been opened and had no sign of having been pre sealed.

Please look at the enclosed picture.