More than that, what goal are you achieving by letting them invest? Will you use that money to hire a full time sales person, or another dev to accelerate the p

More than that, what goal are you achieving by letting them invest? Will you use that money to hire a full time sales person, or another dev to accelerate the product, or...? Because if you don't know what you'd do with that money, you don't want that money (nor the extra voice and trouble it would bring). You can always keep this thing bootstrapped. If you reach a point you need an extra pair of hands, you can evaluate if it makes sense to hire someone, and if debt or equity financing is necessary. But if you're not at that point, there's no reason to take that on. A key thing to remember is being invested in is a means to an end. Ultimately it requires giving up a level of control, or taking on a level of debt; it's not simply 'a good thing to do'. So even without considering the downsides of a customer as a shareholder, you may want to avoid doing it just because you don't know how the money will advance your goals.