Simply replace:
<input type="hidden" name="item_name" value="The product name">
With the following:
<select name="item_name">
<option value="Product name A">Product name A</option>
<option value="Product name B">Product name B</option>
<option value="Product name C">Product name C</option>
</select>
This will add a dropdown from which you can pick a product / item name. This will be subsequently be used as the item throughout the full PayPal transaction.
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