I see that a super market is now rounding all customer purchases

Cape May County - I see that a super market is now rounding all customer purchases up to the next dollar amount. For example, my purchase totaled $17.61, but they charged my credit card $18.00 (39 cents higher). The register receipt showed that the 39 cents was a "donation" (no recipient named). It was very generous for a major grocery store chain to make a donation with my money without asking my permission. I donate to many charities and try to make sure my charities make good use of my money. It seems illegal that a merchant can unilaterally add money to my bill even, if it's for charity.