XDocument _document = XDocument.Parse(XMLString);

<Line>
 <ITEMNMBR>18-4695</ITEMNMBR>
 <UNITCOST>2.00000</UNITCOST>
 <UNITPRCE>7.00000</UNITPRCE>
 <QUANTITY>15.00000</QUANTITY>
</Line>
<Line>
 <ITEMNMBR>18-4695</ITEMNMBR>
 <UNITCOST>3.00000</UNITCOST>
 <UNITPRCE>7.00000</UNITPRCE>
 <QUANTITY>22.00000</QUANTITY>
</Line>

var lines = _document.Descendants("Line")
                  .Select(l => l.Elements().ToDictionary(e => e.Name.LocalName, e => e.Value)).ToList();

var itemNumbers = lines.Where(dict => dict.ContainsKey("ITEMNMBR"))
                    .Select(dict => dict["ITEMNMBR"])
                    .Distinct()
                    .ToList();

foreach (string itemNumber in itemNumbers)
  {...

var qty = lines.Where(dict => dict.ContainsValue(itemNumber))
                    .Select(dict => dict["QUANTITY"])
                    .ToList();

int sum = qty.Sum(x => Convert.ToDecimal(x.Value));

var qty = lines.Where(dict => dict.ContainsValue(itemNumber))
          .Select(dict => dict["QUANTITY"]) // here you are selecting the value
          .ToList();

int sum = qty.Sum(x => Convert.ToDecimal(x));

var sum = lines.Where(dict => dict.ContainsValue(itemNumber))
                .Sum(dict => Convert.ToDecimal(dict["QUANTITY"]));

decimal sum = qty.Sum(x => !string.IsNullOrEmpty(x) ? Convert.ToDecimal(x) : 0);