Probably your least expensive option. There is an office in Geneva (rue de Lausanne) but they will probably need a day to have the vehicle there as I think that they keep most of the trucks close to Lausanne.
In my opinion Patrick Location has some of the cheapest fares, if you can ever find a vehicle, as they go quickly. Ikea as someone else said has great offers and another cheap option is ADA.
ADA Location de Véhicules - www.autolocationleman.ch
rue du Prieuré 12, Genève - +41 22 901 12 62