script.liquid

{% assign purchased_variant = nil %}
{% for line_item in order.line_items %}
  {% if line_item.product_id == options.product_id__required_number %}
    {% assign purchased_variant = line_item.variant %}
    {% break %}
  {% endif %}
{% endfor %}

{% if event.preview or purchased_variant %}
  {% assign next_price = purchased_variant.price | plus: options.increase_price_by_this_much__number_required %}

  {% unless event.preview != true and options.maximum_price__number != blank and next_price > options.maximum_price__number %}
    {
      "action": {
        "type": "shopify",
        "options": [
          "update",
          [
            "variant",
            {{ purchased_variant.id | json }}
          ],
          {
            "price": {{ next_price | json }}
          }
        ]
      }
    }
  {% endunless %}
{% endif %}