Why not, because I would be concerned about the possibility of only getting $1 back in a dispute.  However if I did buy I wouldn't be leaving a neg for excess postage because you told me thats how much it would be and I entered in to the contract fully knowing what the cost would be. 

How would you like it if I bought an item of you with free post for $301 and left a neg because I know the wholesale cost of the item is only $200 so you ripped me off.  The only difference its easy to see what postal costs are but not wholesale costs.

eBay have no way of knowing what the postal costs are to an individual, it is reportable because they want to gouge every cent they can.  Do you believe in the US were postage is included in the FVF that eBay will never increase fees again - I dont think so.