While I have been using the same plastic bag for at least five years, I still get surprised when anyone says that 7-Eleven gives free plastic bags away. 7-11 does not manufacture plastic bags. The plastic bags for 7-Eleven are manufactured by Naraipak, a major plastic bag producer in Thailand. So why do people think the bags are free? Because they don't notice the price is added to the goods that 7-Eleven sells.
Interestingly, when they went through a phase of not handing out bags, the price of their products did not go down accordingly.