You can "hoodwink" zencart into doing this, if you know your times-tables, and/or didn't flunk math in the third grade!
Ajeh has described a technique where you use the product's WEIGHT field as a "container" for the
variable on one side of the equation.
(You need to render the site-side display of "weight" invisible as it's no longer a "weight" value.)
Then you configure your shipping module to calculate the shipping rate by weight, and then apply the
constant for the other side of the equation.
EXAMPLE:
Set the constant in the shipping module to $1 per 1 lb (or whatever value is needed - I use 1 for my example.
Item A shipping is $1.25
(the variable) - (so in the weight field, put 1.25... 1
(the constant)X1.25 = 1.25)
Item B shipping is $2.99
(the variable) - (so in the weight field, put 2.99... 1
(the constant)X2.99 = 2.99)
...etc