Thank you, Steve, I have followed your reply and used those methods, but I'm stuck trying to retrieve the formats from the printer. Could you please give me a hand looking to my methods? 
 

I used ZebraDesigner for developers and stored the format in the internal flash. As a reference, this was the ZPL generated (which I think should be the same stored in the printer):

CT~~CD,~CC^~CT~
^XA
^DFE:SHIPPING.ZPL^FS
~TA000
~JSN
^LT0
^MNW
^MTT
^PON
^PMN
^LH0,0
^JMA
^PR6,6
~SD30
^JUS
^LRN
^CI27
^PA0,1,1,0
^MMT
^PW900
^LL600
^LS0
^FT42,128^A0N,38,38^FH\^CI28^FDSupplier:^FS^CI27
^FT42,215^A0N,38,38^FH\^CI28^FDPart Number:^FS^CI27
^FT42,320^A0N,38,38^FH\^CI28^FDRev:^FS^CI27
^FT42,434^A0N,38,38^FH\^CI28^FDLot:^FS^CI27
^FT293,128^A0N,38,38^FH\^CI28^FN1"SUPPLIER"^FS^CI27
^FT293,215^A0N,38,38^FH\^CI28^FN2"PARTNUMBER"^FS^CI27
^FT293,320^A0N,38,38^FH\^CI28^FN3"REV"^FS^CI27
^FT293,434^A0N,38,38^FH\^CI28^FN4"LOT"^FS^CI27
^BY4,15^FT515,432^B7N,15,2,0,0,N
^FH\^FN5"BARCODE"^FS
^XZ
^XA
^XFE:SHIPPING.ZPL^FS
^CI27^FN1^FH\^FD^FS
^CI27^FN2^FH\^FD^FS
^CI27^FN3^FH\^FD^FS
^CI27^FN4^FH\^FD^FS
^CI27^FN5^FH\^FD^FS
^PQ1,0,1
^XZ

Attached is my method to get the variable fields from the printer. 

 

    Private Sub PrintBarcodeImage()
        Try
            Dim connection As New UsbConnection("\\?\usb#vid_0a5f&pid_00fa#18j152704918#{28d78fad-5a12-11d1-ae5b-0000f803a8c2}")
            connection.Open()

            Dim getZebraUsbPrinters As List(Of DiscoveredUsbPrinter) = UsbDiscoverer.GetZebraUsbPrinters()

            Console.WriteLine(getZebraUsbPrinters)

            Dim printer As ZebraPrinter = ZebraPrinterFactory.GetInstance(connection)

            Dim printerStatus As PrinterStatus = printer.GetCurrentStatus()

            If printerStatus.isReadyToPrint Then
                Console.WriteLine("Ready to print")
            Else
                Console.WriteLine("Not ready to print")
            End If



            Dim formatData As Byte() = printer.RetrieveFormatFromPrinter("E:SHIPPING.ZPL")
            Console.WriteLine("formatdata: " & formatData.ToString)
            Dim formatString = formatData.ToString
            Dim variableFields As FieldDescriptionData() = printer.GetVariableFields(formatString)




            For Each field As FieldDescriptionData In variableFields
                Console.WriteLine("Field number: " & field.FieldNumber)
                Console.Write("Variable name: " & field.FieldName)
            Next


            connection.Close()

        Catch ex As Exception
            MessageBox.Show(ex.Message, "Error", MessageBoxButtons.OK, MessageBoxIcon.Error)
        End Try
    End Sub

    Private Sub FindPrinters()
        Try
            For Each printer As DiscoveredPrinterDriver In UsbDiscoverer.GetZebraDriverPrinters()
                Console.WriteLine(printer)
            Next

            For Each usbPrinter As DiscoveredUsbPrinter In UsbDiscoverer.GetZebraUsbPrinters(New ZebraPrinterFilter())
                Console.WriteLine(usbPrinter)
            Next
        Catch e As ConnectionException
            Console.WriteLine($"Error discovering local printers: {e.Message}")
        End Try

        Console.WriteLine("Done discovering local printers.")
        Console.WriteLine()
    End Sub

Thank you!! 
Eduardo