(1) As provided by 82-15-109 , MCA, liquefied petroleum gas (LPG) shall be sold by liquid measure or weight as determined with a device licensed by the department.  Customer charges for LPG purchase at retail shall be computed only on the basis of the net weight or liquid measure received by the purchaser.  For sales of less than five gallons or 20 pounds, the retailer may charge, in addition to the charge for the LPG, a bottle filling service charge.  If the retailer chooses to bill for a bottle filling service charge, the additional service charge must be clearly disclosed.  The disclosure of the bottle filling service charge shall consist of the retailer's displaying the additional dollar amount for the filling service and the conditions under which the service fee will be charged.  The disclosure shall be displayed in a clear and conspicuous manner on both the dispensing device and on all on-site signs advertising the availability of LPG at the site.  Minimum transaction charges based on the size of the customer's LPG container or set at flat or fixed dollar amounts, without regard for the actual quantity of LPG remaining in the customer's container(s) , are prohibited.