One well-respected auction house used to have a default shipping charge that wasn't changed to the actual shipping charge until hours after the auction closed. Invoices were available for review before the shipping charges were changed.
The default shipping charge for all items was $1000.00. Yikes. That will open your eyes in the middle of the night at 4AM when the auction closed.
My only experience with this practice involved two slabbed cards. The shipping charges were reduced later the same day from $1000.00 to $20.00.
