That is not an error, it is an XML response saying that the edit was successful. But you do not want to expose that to your users.
If you want to refresh the page after the button is clicked, use this syntax.
var text URL = URLRoot() & "db/" & [_DBID_EXPRESS_SHIPPING_TRACKING]
& "?a=API_EditRecord&rid="&[Record ID#]&"&apptoken=bm2czcxzy&_fid_18=Close";
"javascript:" &
"$.get('" &
$URL &
"',function(){" &
"location.reload(true);" &
"});"
& "void(0);"
If you always name your formula variable URL, then that will work.
Here is the help text on what a formula variable is
http://help.quickbase.com/user-assistance/formula_variables.html