The second 'solution' which calculates results based on two input cells and a formula (which is selected based on what is entered in C1) is what the solution I posted does, and it does it using the VLOOKUP function.
This is how it works - using the original example:
A1 and B1 contain user entered values, lets say 21 and 7
Y1 to Z4 contain a list of functions and their respective formulas. This is what the 8 cells look like:
1 SUM 28
2 SUB 14
3 MUL 147
4 DIV 3
and this is what the cells contain:
1 SUM =A1+B1
2 SUB =A1-B1
3 MUL =A1*B1
4 DIV =A1/B1
The results of the four calculations are created by standard formulas using the data from A1 and B1.
Cell Z2 contains =A1-B1 i.e. 21-7 which shows 14.
In C1 the user selects from the list of available functions.
If SUB is selected, the formula in D1 uses VLOOKUP, to find the item in C1 in the column of cells Y1 to Y4, and then returns the value from the adjacent column of cells Z1 to Z4.
When SUB is entered in C1, VLOOKUP finds SUB in cell Y2 and returns the value 14 from cell Z2.
With SUB selected in C1 if the user changes the values in A1 and B1 the result in D1 will change.
As a result I think that the solution I originally posted will work for you. Obviously the formulas will be more complicated and they will refer to more than two cells for the variables, such as thickness, diameter, length ...
but as long as each formula is alongside it's product description, then the user will get the result appropriate for the product description they have selected.