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.