Once you’ve got the flap open, a good spray with silicone lubricant (or smear of silicone grease) on the operating pin/catch and hinge should keep the mechanism dry and hence prevent icing up in future.
Put some grease around the mechanical parts that flip the lid open.
I had this happen to my daily driver, I got the flap open but couldn’t get the locking cap off. The lock would work but water had frozen the cap on. Red rubber grease around it helped chase any water away, never had that problem again.
Most filling stations these days seem to sell hot drinks. Should be able to get a cup of hot water without paying. If they can’t or wont let you have that then try the next one down the road.