Macros, Variables and Includes
You aren't able to pass an import into a macro. So helpers needs to be imported into each macro file that uses it.

You can however pass the cart to your macros. As a side note, it's definitely best to set the cart variable in a global template - this means you're only ever setting it once. All templates that extend this global template then inherit cart. The way macro's work though means they need to have these variables explicitly passed to them. They don't inherit variables in the same way includes do or when you extend templates.

You can then pass cart to your macro like so:

{# Variable set in an extended template or current template #}
{% set cart = craft.commerce.cart %}

{# Variable is passed to the macro #}
{{ teaser(product, imgSize, className, cart) }}


Macro file then edited to take variables:

{% macro teaser(product, imgSize, className, cart) %}
    {# We still need to import our helpers into this macro file #}
    {% import '_macros/_helpers' as helpers %}

    {% set category = product.category.first() %}

    &lt;div class="product-teaser{% if className %} // {{ className }}{% endif %}"&gt;
        {% set thumbAsset = product.productImage.first() %}
        {% if thumbAsset %}
        &lt;a href="{{ product.url }}"&gt;
            &lt;img
                src="{{ craft.imager.transformImage(thumbAsset, { width:imgSize, height:imgSize })}}"
                srcset="{{ craft.imager.transformImage(thumbAsset, { width:imgSize, height:imgSize })}} 1x, {{ craft.imager.transformImage(thumbAsset, { width:imgSize * 2, height:imgSize * 2 }) }} 2x"
                alt="{{ product.title }}"
                class="product-teaser__img"
            /&gt;
        &lt;/a&gt;
        {% endif %}
        &lt;h3 class="product-teaser__title{% if className %} // {{ className }}__title{% endif %}"&gt;{{ helpers.product_title(product.title) }}&lt;/h3&gt;
        &lt;p class="product-teaser__price{% if className %} // {{ className }}__price{% endif %}"&gt;{{ product.defaultPrice|currency(cart.currency) }}&lt;/p&gt;

        &lt;p&gt;
            &lt;a href="{{ product.url }}" class="btn // btn--sub"&gt;View Product&lt;/a&gt;

            {% if className == "product-teaser--featured" %}
                &lt;a href="{{ category.url }}" class="btn // btn--base"&gt;{{ category.title }} Range&lt;/a&gt;
            {% endif %}
        &lt;/p&gt;
    &lt;/div&gt;
{% endmacro %}