Or more specifically: hired a car in Prague, with the view to driving across the border to Poland and back?

Found a few good deals, but things go a little wonky when you mention you’re going across the border. I’ve got one site that’ll give a firm quote, but some of the big companies just say “an extra fee may apply” when you pick up the car – don’t fancy them trying to bend me over for £xx when I arrive at the airport.