Hi,
1. Keep the submit activated and remove the line:
$checked.=' onclick="this.form.action=this.form.action+\'#hikashop_shipping_methods\';this.form.submit(); return false;"';
of the file "shipping" of the view "checkout" via the menu Display>Views.
2. You could change the line:
$method->custom_html='<span style="margin-left:10%">'.JText::_('PURCHASE_ORDER_NUMBER').'<input type="text" class="hikashop_purchase_order_number inputbox required" name="hikashop_purchase_order_number" value="'.@$_SESSION['hikashop_purchase_order_number'].'"/> *</span>';
to:
$method->custom_html='<span style="margin-left:10%">'.JText::_('PURCHASE_ORDER_NUMBER').'<input type="text" class="hikashop_purchase_order_number inputbox required" name="hikashop_purchase_order_number" value="'.@$_SESSION['hikashop_purchase_order_number'].'"/> *</span> <input type="button" value="OK" onclick="this.form.submit();return false;"/>';
in the file plugins/hikashoppayment/purchaseorder/purchaseorder.php