Its not a great idea. Paint is very brittly and effected by temperature. Curriers will leave the paint in a warehouse overnight usually, or in a truck or something which is not heated and the paint may well freeze. If it doesnt break in the box due to being chucked around then it is likely to when ur playing as the shells will have small cracks in them due to freezing.
If you can avoid it get paint from a paintball shop which would be able to deliver themselves, or let you pick it up.
If you are in/around milton keynes there are a couple of sites near you, give them a ring and tell them your situation, im sure they will help.