Either don't say anything (when the person says 
they want to pay by Paypal, you tell them then) 
or you say something like "if you would like to 
use Paypal, you need to contact me after the end 
of the auction", or you simply become stupidly 
unclear on how much the postage is. . .

There are ways around that. The second one is so 
common now, it's become almost as good a warning  
as saying "there is a charge to use Paypal".

Many Sharp Smiles,
--Drac