I have the same gripe about post office mail bags. The only way I got some in the end was to make a formal complaint. You would think that an organisation like that would have better arrangements for the bags this time of the year. I was told to use black bin bags, hum, due to them not providing the sacks I had been using black bin liners anyway!!
My other gripe is when a customer orders an item on eBay to be collected at Argos. The number of complaints I am getting asking where is my item, and every one i have checked has gone via Argos! I am seriously considering not providing that facility.