XSLT transform: Conditional copy of a child node based on a set of criteria

Relatively new to XSLT.

Here’s a sample XML:

<Shipments>
                <Shipment>
                 <ErpOrder>415580479</ErpOrder>
                    <Containers>
                        <ShippingContainer>
                            <TreeUnit>12211620</TreeUnit>
                            <WeightUm>L</WeightUm>
                        </ShippingContainer>
                    </Containers>
                </Shipment>
                <Shipment>
                    <ErpOrder>415580479</ErpOrder>
                </Shipment>
                 <Shipment>
                    <ErpOrder>5124516</ErpOrder>
                </Shipment>
</Shipments>

I want to copy the <Containers> node and all child tag within that node to any <Shipment> tag that matches the <ErpOrder> value and is missing the <Containers> node.

Output:

<Shipments>
            <Shipment>
             <ErpOrder>415580479</ErpOrder>
                <Containers>
                    <ShippingContainer>
                        <TreeUnit>12211620</TreeUnit>
                        <WeightUm>L</WeightUm>
                    </ShippingContainer>
                </Containers>
            </Shipment>
            <Shipment>
                <ErpOrder>415580479</ErpOrder>
                 <Containers>
                    <ShippingContainer>
                        <TreeUnit>12211620</TreeUnit>
                        <WeightUm>L</WeightUm>
                    </ShippingContainer>
                </Containers>
            </Shipment>
             <Shipment>
                <ErpOrder>5124516</ErpOrder>
            </Shipment>
</Shipments>

What I have so far, but I think is completely off [EDITED]:

<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
 <xsl:output omit-xml-declaration="no" indent="yes" method="xml"/>

 <xsl:key name = "erpordermatch" match = "Shipment" use = "@ErpOrder"/>

 <xsl:template match="node()|@*">
    <xsl:copy>
      <xsl:apply-templates select="node()|@*"/>
    </xsl:copy>
  </xsl:template>

<xsl:template match="Shipment[not(Containers)]">
     <xsl:copy>
      <xsl:apply-templates select="node()|@*"/>
      <xsl:for-each select="key('erpordermatch', @ErpOrder)">
            <xsl:if test="@ErpOrder = ErpOrder">
                <xsl:copy-of select="Containers"/>
            </xsl:if>
        </xsl:for-each>
 </xsl:copy>                
            
    </xsl:template>
</xsl:stylesheet>

<xsl:stylesheet version="1.0" 
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="xml" version="1.0" encoding="UTF-8" indent="yes"/>
<xsl:strip-space elements="*"/>

<xsl:key name="ship-by-erp" match="Shipment" use="ErpOrder" />

<!-- identity transform -->
<xsl:template match="@*|node()">
    <xsl:copy>
        <xsl:apply-templates select="@*|node()"/>
    </xsl:copy>
</xsl:template>

<xsl:template match="Shipment[not(Containers)]">
    <xsl:copy>
        <xsl:apply-templates/> 
        <xsl:copy-of select="key('ship-by-erp', ErpOrder)/Containers"/>
    </xsl:copy>
</xsl:template>
  
</xsl:stylesheet>