I'll offer one exception to the "don't work for free" rule: valuable pro-bono.
If you pick your charity well, you'll get kudos from it, and exposure to useful decision makers. Many charities have boards made up of well-off or powerful decision makers in other places that are willing to spend money in their day-jobs, but like you are working pro-bono for the charity.
If you circulate with these people, and deliver a high-quality product, you get the benefit of exposure to the right sort of future customers, the benefit of kudos for pro-bono (in the face of people who are also pro-bono and so value your input as much as they value their own), and you don't have the problem of your product being undervalued, because it's for a charitable good, not a discount for someone who should actually be paying for it.
If you pick your charity well, you'll get kudos from it, and exposure to useful decision makers. Many charities have boards made up of well-off or powerful decision makers in other places that are willing to spend money in their day-jobs, but like you are working pro-bono for the charity.
If you circulate with these people, and deliver a high-quality product, you get the benefit of exposure to the right sort of future customers, the benefit of kudos for pro-bono (in the face of people who are also pro-bono and so value your input as much as they value their own), and you don't have the problem of your product being undervalued, because it's for a charitable good, not a discount for someone who should actually be paying for it.