It clearly states on the click & send site that if you are using your own packaging that the price is not fixed and may differ when you get to the PO.

Some people don't realise that if a box has a slight 'bump' in the middle of one of the sides it will increase the measurement, one cm on a long side can make quite a difference.

On the rare occasions I have used C&S with my own packaging I have added at least 1cm to each of the three dimensions, and allowed for any bumps, I can always give the buyer a refund if it ends up costing less.

It has never cost more than c&s estimated.