You could have revised your listing at any time before it sold. Are you referring to the Lawson clock? That was listed with a flat shipping cost ($18) to any ZIP Code.
The prices for Ground Advantage are not double the old rates for Retail Ground or Parcel Select; some rates went up a little and some went down. The jump in postage cost is probably due to the fact that you set a flat shipping cost that was unrealistically low.
What are the dimensions and weight of your package? How did you decide to set the shipping cost at $18 when you set up the listing?
