When you pick up the box off your porch, you will understand why shipping is $100 for the pair of Progressive Rate and Light Duty Dock Springs.  The Heavy Duty Dock Springs are sold individually with a flat $60 shipping each.  If I were to put them in one box as a pair to ship, the box would weigh about 125 pounds.  Your UPS guy would hate you.
If in doubt about what size you need, call me at 205-966-2137 and I can advise you on which set to get.