I find that my product weight keeps changing when I reach check out. For eg, when I enter product weight as 500grams while product creation when a customer checks out it becomes 980 grams and shows a totally different chipping charge. Anyone had the same problem? This happened at ARRS Silks
I have not experienced this but I will try to give you my 2 cents on why this is happening. I assume your charge for shipping based on the weight of the product? If yes the final price is the weight of the actual product plus the weight of the package you use in shipping. That may increase the final weight and final shipping price.
You set the rate of your default packaging on shipping page under where it says default package. check to see if that is set to 0. if that is not where the problem is coming from reply to this, also if you fix it, let me know how you did it.