I've created my own product document type that inherits the standard 'Product' document type so it has all the correct product properties plus a few extra I need.

After adding the product to the order in my .Net user control with the following test code:

TeaCommerce.Base.AddOrderLine(Node.GetCurrent().Id, 1);

I get the following error: "The '$' character, hexadecimal value 0x24, cannot be included in a name." when the following line in MiniCart.xslt is executed:

<xsl:variable name="order" select="teacommerce:GetOrderXml()" />

Any ideas what this could mean?

System.Xml.Xsl.XslTransformException was unhandled by user code
  Message=An error occurred during a call to extension function 'GetOrderXml'. See InnerException for a complete description of the error.
  Source=System.Data.SqlXml
  LineNumber=0
  LinePosition=0
  StackTrace:
       at System.Xml.Xsl.Runtime.XmlExtensionFunction.Invoke(Object extObj, Object[] args)
       at System.Xml.Xsl.Runtime.XmlQueryContext.InvokeXsltLateBoundFunction(String name, String namespaceUri, IList`1[] args)
       at System.Xml.Xsl.CompiledQuery.Query.<xsl:template match="/">(XmlQueryRuntime {urn:schemas-microsoft-com:xslt-debug}runtime, XPathNavigator {urn:schemas-microsoft-com:xslt-debug}current, Double {urn:schemas-microsoft-com:xslt-debug}position, Double {urn:schemas-microsoft-com:xslt-debug}last, IList`1 {urn:schemas-microsoft-com:xslt-debug}namespaces) in D:\Websites\5starfootballtickets\xslt\miniCart.xslt:line 17
       at System.Xml.Xsl.CompiledQuery.Query.<xsl:apply-templates>(XmlQueryRuntime {urn:schemas-microsoft-com:xslt-debug}runtime, XPathNavigator , Double , Double )
       at System.Xml.Xsl.CompiledQuery.Query.Root(XmlQueryRuntime {urn:schemas-microsoft-com:xslt-debug}runtime)
       at System.Xml.Xsl.CompiledQuery.Query.Execute(XmlQueryRuntime {urn:schemas-microsoft-com:xslt-debug}runtime)
       at System.Xml.Xsl.XmlILCommand.Execute(Object defaultDocument, XmlResolver dataSources, XsltArgumentList argumentList, XmlSequenceWriter results)
       at System.Xml.Xsl.XmlILCommand.Execute(Object defaultDocument, XmlResolver dataSources, XsltArgumentList argumentList, XmlWriter writer)
       at System.Xml.Xsl.XslCompiledTransform.Transform(IXPathNavigable input, XsltArgumentList arguments, XmlWriter results, XmlResolver documentResolver)
       at System.Xml.Xsl.XslCompiledTransform.Transform(IXPathNavigable input, XsltArgumentList arguments, TextWriter results)
       at umbraco.macro.GetXsltTransformResult(XmlDocument macroXML, XslCompiledTransform xslt, Dictionary`2 parameters)
       at umbraco.macro.GetXsltTransformResult(XmlDocument macroXML, XslCompiledTransform xslt)
       at umbraco.macro.loadMacroXSLT(macro macro, MacroModel model, Hashtable pageElements)
  InnerException: System.Xml.XmlException
       Message=The '$' character, hexadecimal value 0x24, cannot be included in a name.
       Source=System.Xml
       LineNumber=0
       LinePosition=6
       StackTrace:
            at System.Xml.XmlConvert.VerifyNCName(String name, ExceptionType exceptionType)
            at System.Xml.Linq.XName..ctor(XNamespace ns, String localName)
            at System.Xml.Linq.XNamespace.GetName(String localName)
            at System.Xml.Linq.XName.Get(String expandedName)
            at System.Xml.Linq.XName.op_Implicit(String expandedName)
            at TeaCommerce.Data.OrderProperty.GetXElement()
            at TeaCommerce.Data.OrderLine.<GetXElement>b__d(OrderLineProperty i)
            at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()
            at System.Xml.Linq.XContainer.AddContentSkipNotify(Object content)
            at System.Xml.Linq.XElement..ctor(XName name, Object content)
            at TeaCommerce.Data.OrderLine.GetXElement()
            at TeaCommerce.Data.Order.<GetXElement>b__68(OrderLine i)
            at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()
            at System.Xml.Linq.XContainer.AddContentSkipNotify(Object content)
            at System.Xml.Linq.XContainer.Add(Object content)
            at TeaCommerce.Data.Order.GetXElement()
            at TeaCommerce.Data.Order.GetXml()
            at TeaCommerce.Library.GetOrderXml()
       InnerException: