This description could imply that the OP could bill more than $150 only, so I wouldn't call this offer too cheap or suspicious because anyone would ask for more customized features than the feature descriptions offered in the end.
However, escrow is recommended in this agreement to be more secure.
as mentioned before i always use a middleman, for both of our safety, yes the script is for $150 but if you want any change on it exclusively of course that would cost you extra