Hi Ivan
There are the two files                         
They have sample date for a three line invoice.
The invoice could have up to 15 lines in which case
the source would have fields Quantity01...Quantity15 etc

I have reduced the number of elements to what I 
feel is required to get the point across
---------------------- Source File -------------
<?xml version="1.0" encoding="UTF-8"?>
<topmostSubform>
   <InvoiceNumber>1020555.00</InvoiceNumber>
   <CustomerName>ABC Exports</CustomerName>
   <Quantity01>5</Quantity01>
   <Description01>Big Western New York Snow Shovels</Description01>
   <UnitPrice01>120.00</UnitPrice01>
   <Amount01>600.00</UnitPrice01>
   <Quantity02>10</Quantity02>
   <Description02>Very Warm Gloves</Description02>
   <UnitPrice02>20.00</UnitPrice02>
   <Amount02>200.00</UnitPrice02>
   <Quantity03>20</Quantity03>
   <Description03>Bottles of Brandy</Description03>
   <UnitPrice03>40.00</UnitPrice03>
   <Amount03>800.00</UnitPrice03>
   <TotalAmount>1600.00</TotalAmount>
</topmostSubform>
---------------------- Destination File -------------
<?xml version="1.0" encoding="UTF-8"?>
<Invoice>
   <InvoiceNumber>1020555.00</InvoiceNumber>
   <CustomerName>ABC Exports</CustomerName>
   <InvoiceLine>
      <Quantity>5</Quantity>
      <Description>Big Western New York Snow Shovels</Description>
      <UnitPrice>120.00</UnitPrice>
      <Amount>600.00</UnitPrice>
   </InvoiceLine>
   <InvoiceLine>
      <Quantity>10</Quantity>
      <Description>Very Warm Gloves</Description>
      <UnitPrice>20.00</UnitPrice>
      <Amount>200.00</UnitPrice>
   </InvoiceLine>
   <InvoiceLine>
      <Quantity>20</Quantity>
      <Description>Bottles of Brandy</Description>
      <UnitPrice>40.00</UnitPrice>
     <Amount>800.00</UnitPrice>
   </InvoiceLine>
   <TotalAmount>1600.00</TotalAmount>
</Invoice>