am wondering if LE sets up a website for sting can they legally put donation amounts on the site? I know nothing about legalities thats why am asking.

it's up to the DA and the courts to decide if LE's behavior is legal and therefor whether or not there's a case against you. How many times do you see busts thrown out of court.

there can never be a 100% answer.   There is no 100% fool proof way to beat the system except to play in a jurisdiction where it is legal.  In the opinion of many, the best protection is to see well reviewed providers who take measures to protect themselves & their clients.   LE normally focuses on easy targets:  AMPs with obivious street presence.  Providers who accept unscreened clients on short notice.
However, targeted investigations are sometimes the response to complaints...  neighbors, hotels, angry spouces.   The name of the game is to always be discreet to minimize what risk there is.  

It is generally legal (though sometimes license required) to say request a fee for modeling.  All the legality disappears is superceded by new facts when the sex for money line is crossed.  
skb