POST data is not within the http headers. And it's also not a term used for a http repsonse. A http request may use the POST method and therefore have POST data appended. The other way round it's almost the same but it's not called POST anyway

Let's what request my browser sends to devnetwork.net when I hit the preview button right now.
POST /posting.php HTTP/1.1
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; de; rv:1.8.1) Gecko/20061010 Firefox/2.0
Accept: text/xml,application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5
Accept-Language: en-us,en;q=0.8,de-de;q=0.5,de;q=0.3
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
Content-Type: application/x-www-form-urlencoded
Content-Length: 592
subject=a&addbbcode100=-1&addbbcodefontcolor=%23444444&addbbcodefontsize=12&helpbox=Font+color%3A
+%5Bcolor%3Dred%5Dtext%5B%2Fcolor%5D++Tip%3A+you+can+also+use+color%3D%23FF0000&message=POST+data+
is+not+within+the+http+headers.+And+it%27s+also+not+a+term+used+for+a+http+repsonse.+A+http+request
+may+use+the+POST+method+and+therefore+have+POST+data+appended.+The+other+way+round+it%27s+almost
+the+same+but+it%27s+not+called+POST+anyway+%3B%29%0D%0ALet%27s+what+request+my+browser+sends+to+devnetwork.net
+when+I+hit+the+preview+button+right+now.&attach_sig=on&mode=reply&t=59272&preview=Preview
(removed some headers, added some line breaks)
You see it sends the headers then a blank line. Everything after that first blank line is the request body (not the request header anymore). According to the request method (
POST /posting.php HTTP/1.1) it's POST data and according to
application/x-www-form-urlencoded it's urlencoded (e.g. space->+).
And the server send a http response:
Code: 200 - OK
Set-Cookie: phpbb2mysql_data=[...]; expires=Tuesday, 20-Nov-07 17:21:45 GMT; path=/; domain=.devnetwork.net
phpbb2mysql_sid=[...]; path=/; domain=.devnetwork.net
Cache-Control: private, pre-check=0, post-check=0, max-age=0
Content-Type: text/html
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html dir="ltr">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<meta http-equiv="Content-Style-Type" content="text/css">
again there are the headers, a blank line and the payload data.
Your "ecommerce driven process" seems to want a POST request with the xml data as request body and also sends back a xml document.