Hi, 
    My wife  and went threw this. We sell at a small farmers market on sat morning.  They never like the belts we have laying there....so we ask them what they want and make  it.  Many of  them have been  the  wrong size so now we take the belt they are wearing and measure it and  then its not fool proof,  we  had them place  the belt on thier  belly rather than below then complaine  that  its not large enough. Now we  make the belt  a little  longer than what they need and use the center hole measurement that  they are  using which gives them a little leway.    
I have no idea how a person can sell belts  over the  internet.  I am sitting there and still fight to get it right.  I  have the book that Al wrote on Belts  and its  great and thats the way we measure.  
Ok I better shut up.. just wanted to put in my two cents  so you don't feel bad for dealing with this, your not the only one that has went down this road. 
later, my friend 
Larry