They keep debiting my account

I am a client of courier guy. I have a registered account with them for shipping parcels. Unfortunately, my experience with them is always terrible. They give a quote for a certain amount and after I pay for the quote, a few days later they debit my account again. This has happened on a number of occasions. When I query this with them, they keep telling me that they made an error with the weight or measurements. I sent an item that weighed 7kgs and they charged me for 92kgs! They are absolutely hopeless.