You don’t need an extra charge to cover packaging costs. It will only make your customer’s think you are nickel and diming them.
While you do have to pay for packaging to put your to-go food in, there are many expenses you are avoiding by not servicing that customer inside your restaurant. There are no chemical costs to washing their dishes, cleaning their table, and cleaning up after any smudges, smears, spills and stains they may leave behind. There are no products used in your bathroom, or condiments at the table. They are not filling a seat which would reduce the number of customers who can sit in your restaurant at one time. There is no wear and tear on your furniture, fixtures or equipment, and no utilities used on their behalf. You do not have to pay the labor for a staff member to service that person in your restaurant, to clean up after them, or perform extra closing duties due to them.
While it may seem to you that to-go packaging is an added expense to servicing takeout customers, it’s really just the opposite. Even with paying extra for packaging, your gross profit on a carryout customer is significantly greater than a dine-in customer.
Instead of charging extra for special carryout containers, you should be concentrating on marketing the fact that you are environmentally conscious, and be secure in the fact that it is making you a larger profit.