I would recommend sealing one end prior to filling. That way even if it is something loose like a powder it doesn't fall out.

The best way I have found to seal straws is to go out and get a cheap set of plyers. Needle nose work best. I'm talking the dollar 98 junk. Fold the straw over and hold in place. Any method will do. Heat the jaws of the plyers. A gas range works real well but even an electric will work. Then close the jaws over the straw. hold until it melts. Keep holding untill it cools enough to hold. The first few times you try you will get them to hot and melt holes. Then you will go the other way and not get them hot enough to make a seal. With a little practice you will get the hang of it and get the heat just right.