JSFiddle - React, Tailwind, and code Playground

HTML

<p id="FinalPriceParra"></p>
<form>Enter Purchase Price
    <input type="text" id="TextInput" value="" />
    <br/>
    <input type="button" value="Submit" onclick="AddShipping()" />
</form>

JavaScript

function AddShipping() {
 
    var PurchasePrice = +document.getElementById('TextInput').value;
    var Shipping;
    if (PurchasePrice <= 25) {
        Shipping = 1.5;
    } else {
        Shipping = PurchasePrice * 10 / 100;
    }
    var FinalPrice = PurchasePrice + Shipping;
    alert(FinalPrice);
}