Ask a few more questions about how it works first.

You're the type of guy who would, upon finding a genie and being informed of their rules, promptly wish for more genies just the get the most bang out of your buck.

"And once I make a decision I just put the collar on her neck? How does that do anything?" you ask.

The shopkeeper gives you an exasperated look. "Really man? You receive a gift that will give you the dream companion of a lifetime and all you want to do is to understand the finer details of how it works?"

You grin sheepishly, "I just want to know my limitations, that's all."

The shopkeep sighs and rubs his forehead. "Alright. Here's what I know. The collar has a spell on it that fires in two stages. The first stage has already been accomplished, you might not have noticed that it tied itself to your signature the moment you picked it up out of the box. The second stage triggers once it has been physically clasped around a living object. Once it detects that event it binds that living thing to you."

"Binds in what way?" You ask.

The shopkeeper groans and sits down behind the counter. "Why do I always get the munchkins?" He digs around under the counter until he finds a thick manilla folder full of an equal measure of technical blueprints and eldritch diagrams. He plops it down on the counter in front of you. "There! Here's the whole thing. Peruse at your leisure. Keep in mind though the store closes at 6pm sharp and we might not be in your area again for a long time. Let me know when you want to actually do something interesting." He gestures to the bell by the counter and heads to the backroom.

The folder looks quite intimidating. You aren't sure if you are going to be able to understand it in time to make a decision but you have the feeling that if you spent a few hours you could learn a few things at the expense of not having much time to shop around later. But a few ideas about how to turn this into your favor are already materializing. What would happen if you put the collar on yourself? Would you be able to command yourself to do anything you want? Perfect willpower and self-control? That sounded kinda sweet. Or maybe you could call the shopkeeper back, quickly get it around his neck and then he'll give you free range over the whole shop. There must be a few more of these things in the backroom, maybe you could get a few more!

So what will it be? Try to read the manual? Collar your own neck? Try to get the shopkeeper under your control? Or call him back and go shopping like a normal customer?