Your client just wants their business problem solved. If they are a reasonable client, unless they made it very clear to you up front, they do not want to be in the business of designing, implementing, testing, distributing, supporting, and maintaining supply chain management applications.
They want order management capabilities they don't have. They want to get those capabilities without hiring 1-3 more full-time people, each at a fully loaded cost of between $120-250k, all of them with ramp-up time and each with a 33+% chance of not working out.
Solve their business problem, collect your rate, and feel comfortable that you've generated a win-win. Get your contracts reviewed by a lawyer (you should keep a lawyer on hot standby for contract review all the time) and make sure that you retain IP. There are lots of different structures that accomplish that.
It is a good thing that after completing this engagement, you will be well positioned to offer this service to a next client, and a next, and even "all" the clients at scale using a SaaS offering. It's what makes you a valuable consultant. You should not be entertaining the idea of giving your services away simply to capture the side effect of getting better at delivering supply chain products afterwards.
Incidentally: something that I am continuously learning the hard way, even as I tell myself every damn time that I know this, but then proceed to make exactly the same mistake again and again: free has a negative signaling value in consulting. Don't do free. Free is terrible. It scares buyers, generates more uncertainty than it dispels, and it sets you up to fail.
By all means, help people out gratis, do favors, be accessible. But when your client needs something done, put a price tag on it or pass the work up.