I've been charged way more than i received weekly since they stopped taking it back. Two weeks ago 15 bags and i got only 8. Last week 14 bags and i got 7! It has been going on since before but it was only 1-2 bags less.
It didn't even occur to me that they might be doing this 
So along with checking the shopping when it arrives to make sure nothing is missing, checking the use by dates to make sure they haven't once again given me two things that both expire the same day they arrive, and also monitoring the offers to try to make sure they honour them, we have to count the bags too!