<items>
    <product>x</product>
    <product>y</product>
    <product>z</product>
    <product>a</product>
</items>

<xsl:for-each select="product">
  <xsl:value-of select="."/>
  <xsl:if test="position() != last()">
     <xsl:text>, </xsl:text>
  </xsl:if>
</xsl:for-each>