$//$ SO function $($assuming this is a typo, and it's meant to be some setup function$)$

document.addEventListener$(\text{'}$DOMContentLoaded$\text{'},$ function$()\{$

$//$ Add an event listener to the Calculate button

document.getElementById$(\text{'}$calculateBtn$\text{'}).$addEventListener$(\text{'}$click$\text{'},$ calculate$)$;

$//$ Move focus to the product cost text box

document.getElementById$(\text{'}$productCost$\text{'}).$focus$()$;

$\})$;

function calculate$()\{$

$//$ Parse the product cost

var productCost $=$ parseFloat$($document$.$getElementById$(\text{'}$productCost$\text{'}).$value$)$;

$//$ Check if the product cost is a valid number and greater than zero

if $(!$isNaN$($productCost$)$ && productCost $>0)\{$

$//$ If valid, calculate the shipping

var totalCost $=$ calculateShipping$($productCost$)$;

$//$ You might want to display the total cost somewhere on your page

$//$ For example: document.getElementById$(\text{'}$totalCost$\text{'}).$textContent $=$ totalCost.toFixed$(2)$;

$\}$ else $\{$

$//$ If not valid, throw an alert

alert$(\text{'}$Please enter a valid product cost greater than zero.$\text{'})$;

$//$ Leave focus on the product cost

document.getElementById$(\text{'}$productCost$\text{'}).$focus$()$;

$\}$

$\}$

function calculateShipping$($productCost$)\{$

var shippingCost;

var shippingRate;

$//$ Define shipping rates based on product cost ranges

if $($productCost $<=50)\{$

shippingRate $=0\mathrm{.}1$; $//10\%$ of product cost

$\}$ else if $($productCost $<=100)\{$

shippingRate $=0\mathrm{.}07$; $//7\%$ of product cost

$\}$ else if $($productCost $<=200)\{$

shippingRate $=0\mathrm{.}05$; $//5\%$ of product cost

$\}$ else $\{$

shippingRate $=0\mathrm{.}02$; $//2\%$ of product cost

$\}$

$//$ Calculate shipping cost

shippingCost $=$ productCost $*$ shippingRate;

$//$ Return the sum of product cost and shipping cost

return productCost $+$ shippingCost;

$\}$