Per this topic, viewtopic.php?f=42&t=13370&p=57606#p57606), some PageHelper.cs methods won't work on pages with UpdatePanels because they use control.Page.ClientScript.RegisterClientScriptBlock instead of the more robust ScriptManager.RegisterClientScriptBlock. Changing this in a future version would make these methods more generally available.
In the referenced topic, this may be why One Page Checkout didn't have a counter for the ship message and just truncated the number of characters to 200.