You can add a Buy X for Total product promotion to a campaign.
The total-discounts element represents the Buy x for total promotion type. The total-discounts element is of type TotalProductDiscount within the existing promotion import or export file.
The following is sample XML:
<promotion campaign-id="Demo Campaign" promotion-id="PP_20OFF">
<enabled-flag>true</enabled-flag>
<qualifiers>
<customer-groups/>
</qualifiers>
<rank>10</rank>
<exclusivity>no</exclusivity>
<name xml:lang="x-default">ShirtsForTotalPrice</name>
<callout-msg
xml:lang="x-default">Buy 2 shirts for $20 total or 3 shirts for $23 total</callout-msg>
<product-promotion-rule>
<!-- only discounted products supported for new promotion type -->
<discounted-products>
<included-products>
<condition-group>
<category-condition catalog-id="product-catalog">
<category-id>Shirts</category-id>
</category-condition>
</condition-group>
</included-products>
</discounted-products>
<total-discounts>
<discount>
<total-fixed-price>20.00</total-fixed-price>
<eligible-quantity>3</eligible-quantity>
</discount>
<discount>
<total-fixed-price>30.00</fixed-price>
<eligible-quantity>5</eligible-quantity>
</discount>
</total-discounts>
</product-promotion-rule>
</promotion>