  We consider the Stackelberg shortest-path pricing problem, which is defined
as follows. Given a graph G with fixed-cost and pricable edges and two distinct
vertices s and t, we may assign prices to the pricable edges. Based on the
predefined fixed costs and our prices, a customer purchases a cheapest s-t-path
in G and we receive payment equal to the sum of prices of pricable edges
belonging to the path. Our goal is to find prices maximizing the payment
received from the customer. While Stackelberg shortest-path pricing was known
to be APX-hard before, we provide the first explicit approximation threshold
and prove hardness of approximation within 2-o(1).
