OSB - XQuery - line 24, col 1 {err}FORG0005: expected 1 item, got 2 or mor

Question

In the below XQ, I am trying to learn making calls to local functions. I have tweaked my earlier XQ, which had a local variable defined [in xf:getCost() function] as XPath to Quantity. Now, I have modified the XQ to make a call to xf:getQty() from xf:getCost(), to get the desired Quantity.
On executing the below code I get Error occurred while executing XQuery: line 24, column 1: {err}FORG0005: expected exactly one time, got 2+ items. Either the XQuery is invalid or .... Request Help
xquery version "1.0" encoding "Cp1252";
(:: pragma parameter="$anyType1" type="xs:anyType" ::)
(:: pragma type="xs:anyType" ::)
declare namespace xf = "http://tempuri.org/purchaseOrder/";
declare function xf:getCost()
as xs:double {
let $po := <purchase-order>
<red-tape/>
<order-item product="p010" price="10.50" quantity="3"/>
<order-item product="p020" price="18.10" quantity="8"/>
<order-item product="p020" price="11.10" quantity="8"/>
</purchase-order>
let $s1 := $po/order-item/@price
let $s2 := $po/order-item/@quantity
let $s3 := sum(for $el at $i in $s1 return $s1[$i] * xf:getQty($i))
return $s3
declare function xf:getQty($index as xs:decimal)
as xs:decimal {
let $po := <purchase-order>
<red-tape/>
<order-item product="p010" price="10.50" quantity="3"/>
<order-item product="p020" price="18.10" quantity="8"/>
<order-item product="p020" price="11.10" quantity="8"/>
</purchase-order>
return ($po/order-item/@quantity[$index])
xf:getCost()

Answer 1

return ($po/order-item[$index]/@quantity)
not
return ($po/order-item/@quantity[$index])