You could always just package it and take it to the post office where...
1) you can stand in line and wait for the worker to weigh it and get an exact weight and pay exact shipping
2) some post offices have a DIY shipping machines if you don't want to bother waiting and just follow the on screen prompts and pay the exact shipping
Another option would be going on Amazon and looking up the games you are shipping and go to the details area of the page and it should say the weight of the game. Might want to add a few ounces just in case on what you are using to package the games.