If you have incorrect data (larger/heavier) in your prefilled weight and/or dimension fields, your eBay label will always be more expensive than the price you get using the correct details.

A simple keystroke error can add heaps to postage charges - eg. For a 0.8kg parcel, if the decimal point is missed on data entry, it becomes an 8kg parcel... but if you pay for postage over the counter, you'll only be charged for an 800g parcel