CUSTOMERS who buy pounds 150- worth of French francs through a Thomas Cook agency will receive a voucher entitling them to two flights for the price of one on any of Air France's eight direct routes to France. The vouchers are valid on Eurobudget, Full Economy and Le Club fares. For bookings made before 31 May, the voucher is also valid against lower Apex and Discover France fares on all direct routes except London to Paris.