When liquefied petroleum gas is offered for sale, sold or delivered in this state in tanks, cylinders or other approved containers, it shall be considered a package except when sale to a consumer is made on a consumption basis. Such sale or delivery on a package basis shall be by net weight which shall be determined by weighing the tank, cylinder or container and its contents before and after filling on scales of approved design and sealed by a sealer of weights and measures. There shall be plainly marked, on the tank, cylinder, container or attached appurtenances, the name or registered symbol of the filling plant, the weight of the empty tank, cylinder or container, including all permanent attachments, but not the valve protecting cap, and the net weight of the contents expressed in avoirdupois pounds.