as of v2.11.5.4 you can do the following instead of editing the html email template directly.
in your theme’s functions.php:

for the main header:

/**customise the first header tr in html emails - typically the site title **/
add_filter('wppizza_filter_htmlemail_head', 'my_html_email_header_filter',10,6);
function my_html_email_header_filter($headertr, $styles, $labels, $transactiondate, $gatewaylabel,$transactionid) {
/*
@headertr			=>	string/tr to output
@styles				=>	array of style declarations. you can use add_filter(wppizza_filter_html_email_style, my_style_filter) on those too
@labels				=>	array of various lables set in localization you could use [not used by default here]
@transactiondate	=>	str of transaction date [not used by default here]
@gatewaylabel		=>	str of gatewaylabel [not used by default here]
@transactionid		=>	str transactionid [not used by default here]
*/
/*
by default this would output the following
$headertr='<tr><td colspan="2" style="'.$styles['mailPadding']['30'].';text-align:center;'.$styles['mailHeaderBackgroundImage'].';background-color:'.$styles['mailHeaderBackgroundColor'].';"><h1 style="font-size:160%;color:'. $styles['mailHeaderTextColour'].';">'.get_bloginfo().'</h1></td></tr>';
*/
/*
in it's simplest form you could do this instead
*/
$headertr='<tr><td colspan="2" style="/*[some-styles like margin,padding, background etc etc]*/">[some text]</td></tr>';
return $headertr;
}

for the info underneath the main header, before client/customer details

/**customise the second to fifth header tr in html emails -  typically some tex label, and other order information**/
add_filter('wppizza_filter_htmlemail_head_info', 'my_html_email_header_info_filter',10,6);
function my_html_email_header_info_filter($infotr, $styles, $labels, $transactiondate, $gatewaylabel,$transactionid) {
/*
@infotrs			=>	array of trs to output
@styles				=>	array of style declarations. you can use add_filter(wppizza_filter_html_email_style, my_style_filter) on those too
@labels				=>	array of various lables set in localization you could use
@transactiondate	=>	str of transaction date
@gatewaylabel		=>	str of gatewaylabel
@transactionid		=>	str transactionid
*/
/*
by default this is made up of the following
$infotr['orderlabel']='<tr><td colspan="2" style="'.$styles['mailPadding']['20x0x0x15'].'">'.$labels['order_details'].'</td></tr>';
$infotr['transactiondate']='<tr><td colspan="2" style="'.$styles['mailPadding']['2x0x0x15'].'">'.$transactiondate.'</td></tr>';
$infotr['paymentdetails']='<tr><td colspan="2" style="'.$styles['mailPadding']['2x0x0x15'].'">'.$labels['order_paid_by'].' '.$gatewaylabel.' ('.$transactionid.')</td></tr>';
$infotr['devider']='<tr><td colspan="2" style="'.$styles['mailDivider'].'">&nbsp;</td></tr>';
*/
/*
so if , for example, you want to add some text after the transaction date (i.e the second tr here, you could do the following
*/
$infotr['transactiondate']='<tr><td colspan="2" style="'.$styles['mailPadding']['2x0x0x15'].'">'.$transactiondate.' [my additional text]</td></tr>';
/*
or if you want to insert another row after the order label
*/	
$infotr['orderlabel'].='<tr><td colspan="2">[my additional row]</td></tr>';
/*
or if you want to get rid of the devider
*/	
unset($infotr['devider']);
return $infotr;
}