shipping_order_advice_instruction_fulfill.xml

<shipping_order_advice_request xmlns=“urn:demandware.com:bf:api:core:v1” xmlns:i=“http://www.w3.org/2001/XMLSchema-instance” i:dummy=“”>
  <instructions>
    <instruction i:type=“shipping_order_advice_instruction_fulfill”>
      <items>
        <item>
          <key>
            <id>item_id</id>
          </key>
          <quantity>qty</quantity>
        </item>
      </items>
      <tracking_number>tracking_number</tracking_number>
      <suppress_notifications>true</suppress_notifications>
    </instruction>
  </instructions>
  <key>
    <id>shipping order id</id>
  </key>
</shipping_order_advice_request>

@jparbros in the doc suppress_notifications is an enum none or customer, can you check on your side what value your are passing ?