Rule of thumb: When in doubt, always use "werden", it's much more common in German. If there's a "von" part in the sense of "by somebody", that's also a strong hint that "werden" is correct, because in most cases, like in your example, this means that a process is described. (There are exceptions.) Also, a complicated verb like "zur Verfügung stellen" is a strong hint in the direction of a process being talked about.
A consideration that I'd like to add to your example is the order of time. You could say:
"Will der Kunde die Ware nicht behalten, muss er ein Formular ausfüllen, das ihm vom Händler zur Verfügung gestellt wird."
Or you could use the perfect tense to stress that the buyer already has the form when they need to fill it out:
"Will der Kunde die Ware nicht behalten, muss er ein Formular ausfüllen, das ihm vom Händler zur Verfügung gestellt worden ist."
Note that this is still a form of "werden". To have been provided with something is not seen as a state in the sense of this rule, the sentence is still talking about the process of providing the form, although it does so in the past.
Let's look at some examples of "Zustandspassiv" with "sein":
"Das Haus ist blau gestrichen." (The house is painted blue.)
"Ich bin total geplättet!" (I'm totally floored!)
These clearly talk about current state or qualities of the house or the person respectively, the processes of painting of "flooring" in the past pale and become insignificant. As soon as you begin to talk about these processes more, you need to switch to "werden":
"Das Haus ist vor langer Zeit von meinem Großvater mit guter Farbe blau gestrichen worden." (The house was painted blue by my grandfather a long time ago with good paint.)