how to set shipping costs for speakers?

Hi folks, I'm trying to sell a pair of speakers for the first time.  I have the original shipping boxes, how should I enter in the dimensions to factor in two boxes worth of shipping costs here on Audiogon listing?  How do you folks handle double pkg. costs, any seller actually does double pkg. on top of original boxes?  Thanks for helping a newbie seller out.

P.S. and yes email sent to Audiogon customer service too