JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://code.jquery.com/jquery-2.2.4.min.js"></script>
<form id="buyForm" action="https://111.ru/api/partners/v1/lightweight/create" method="post">
  <input id="buyFormShopId" name="shopId" value="" type="hidden"/>
  <input id="buyFormSum" name="sum" value="12490" type="hidden">
</form>

<div class="item"> 
  <span>Цена - 12 490p</span><span class="oldPrice">14700</span><a href="#" class="itemBuy" data-id="e15b71db" data-sum="12490">Купить</a>
</div> 

<div class="item"> 
  <span>Цена - 10 490p</span><span class="oldPrice">12700</span><a href="#" class="itemBuy" data-id="e15b71db" data-sum="10490">Купить</a>
</div> 

<div class="item"> 
  <span>Цена - 14 490p</span><span class="oldPrice">16700</span><a href="#" class="itemBuy" data-id="e15b71db" data-sum="14490">Купить</a>
</div>

CSS

#buyForm {
  display: none;
}

.item {
  margin-bottom: 10px;
}

.item span {
  display: inline-block;
  margin: 0 5px;
}

.oldPrice {
  color: rgb(229, 55, 52);
  font-size: 1.2em;
  text-decoration: line-through;
}

.itemBuy {
  color: black;
  text-decoration: none;
  background: #ccc;
  padding: 3px 5px;
  border-radius: 3px;
}

JavaScript

$(document).ready( function() {
    $('.itemBuy').click( function(e) {
        e.preventDefault();
        $('#buyFormShopId').val( $(this).data('id') );
        $('#buyFormSum').val( $(this).data('sum') );
        //--Убираем эту строку
        alert( $('#buyForm').serialize() );
        //--А эту добавляем
        //$('#buyForm').submit();
    })
});