Why would the status of my account change without ...

Why would the status of my account change without my knowing it..or without been informed....if you set my account that I pay on ship request...what happens if my final shipment weight is less than what it was when I booked....what happens to the difference? Is it my loss?...customer service staff informed me yes...it's my loss...such a pity...if someone like me without any history of owing MYUS 1 penny could be so treated...am speechless ...no further comment.