I'm a native French speaker.
In recent years, there has been a new concept of price in French that is "prix libre". A literal translation of "prix libre" is "free price".
This concept means that a seller proposes that the customer has to decide what amount of money he is willing to give for a given good or service. It especially applies to some cultural events. It implies that you may purchase something with a "prix libre" for free (0$/€/...), but the seller encourages the customer to pay something. It is not really seen as a "tip".
My goal is to put this expression on a website where we propose to download a document at "prix libre".