Aptamil 3 big cartons

Has anyone used the big litre aptamil cartons when travelling? I am going Greece next week and I was thinking to take a big carton for the plane journey. My LO has 2 10oz bottles throughout the day, some times 3 oz in afternoon. I thought this would make more sense to put in his bag rather than 4 small cartons. It says on it, once opened keep in a cool dry place and use within 72 hours. So once I open it, if I keep it in his bag, that will be okay right ? He's 15 months and it's my first time travelling with him, trying to think of ways to reduce over packing and space.