I need a form that calculates prices based on input values

Existing js calculates prices for salt based on salt type, pack weight, qty. I need the script changed to follow slightly different rules. Old script allows any qty in any weight. New script needs to restrict quantities to the following: 6 x 25kg Bag Tablet Salt - £88 10 x 25kg Bag Tablet Salt - £120 20 x 25kg Bag Tablet Salt - £200 7 x 10kg Bag Tablet Salt - £80 14 x 10kg Bag Tablet Salt - £107 21 x 10kg Bag Tablet Salt - £136 7 x 8kg Packet Block Salt - £84 14 x 8kg Packet Block Salt - £100 24 x 8kg Packet Block Salt - £145 Delivery options in the old script are not needed. Granular salt is not offered.