JSFiddle - React, Tailwind, and code Playground

HTML

<form id="invoiceForm" method="post">
<label for="customer_id">ID zákazníka:</label><br>
<input type="text" id="customer_id" name="customer_id"><br>

<label for="payment_method">Způsob platby:</label><br>
<select id="payment_method" name="payment_method">
<option value="cash">Hotově</option>
<option value="card">Kartou</option>
<option value="transfer">Převodem</option>
</select><br>

<hr>

<div id="new_item">
<div class="item">
<label for="item_name">Název položky:</label>
<input type="text" class="item_name" name="items[][name]"><br>
<label for="item_quantity">Počet kusů:</label>
<input type="number" class="item_quantity" name="items[][quantity]"><br>
<label for="item_price">Cena za kus bez DPH:</label>
<input type="number" step="0.01" class="item_price" name="items[][price]"><br>
<label for="item_price_vat">Cena za kus s DPH:</label>
<input type="number" step="0.01" class="item_price_vat" name="items[][price_vat]"><br>
<label for="item_vat">DPH:</label>
<select class="item_vat" name="items[][vat]">
<option value="0.21">21%</option>
<option value="0.10">10%</option>
<option value="0">0%</option>
</select><br>
<label for="item_total">Celková cena položky bez DPH:</label>
<input type="number" step="0.01" class="item_total" name="items[][total]" readonly><br>
<label for="item_total_vat">Celková cena položky s DPH:</label>
<input type="number" step="0.01" class="item_total_vat" name="items[][total_vat]" readonly><br>
<input type="hidden" class="removeItem" value="Odebrat položku"><br>
</div>
</div>

<input type="button" id="addItem" value="Přidat položku">

<hr>

polozky pro fakturu
<div id="cart">
</div>

<hr>

<label for="total">Celková cena bez DPH:</label><br>
<input type="number" id="total" name="total" readonly><br>
<label for="total_vat">Celková cena s DPH:</label><br>
<input type="number" id="total_vat" name="total_vat" readonly><br>
<label for="rounding">Zaokrouhlení:</label><br>
<input type="number" id="rounding" name="rounding" readonly><br>
<label...