Since that page is loaded via an XmlPackage, you can override the SE tags via the meta templates within the XmlPackage.
Ok cool... found the codes but im not sure what should i replace it with..
I should add the codes here right?
<xsl:template match="/">
<xsl:param name="MetaDescription">??
Correct, you want to add code there to overwrite/set the Meta Description...it should be conditional as needed and fall back to the cart's defaults etc