Simple State Sales Tax Calculator
Mathematica, 112 103 77 76 66 bytes
Mathematica has a builtin for everything
NumberForm[Interpreter[&quot;USState&quot;][#]@&quot;StateSalesTaxRate&quot;#2,{9,2}]&amp;

Takes a state name (any format; abbreviation or full name) and the dollar amount.
Try it on Wolfram Sandbox
Usage
f = NumberForm[Interpreter[&quot;USState&quot;][#]@&quot;StateSalesTaxRate&quot;#2,{9,2}]&amp;

 
f[&quot;CA&quot;, 1025.00]


61.50

f[&quot;miNnNesToA&quot;, 123.45]


8.49

Explanation
Interpreter[&quot;USState&quot;][#]

Interpret the input as a US State name and generate an Entity.
... @&quot;StateSalesTaxRate&quot;

Get the sales tax rate.
... #2

Multiply that by the second input.
NumberForm[..., {9,2}]

Format the result into a number with 9 digits to the left of the decimal and 2 digits to the right.