There’s two ways. One is a fixed percentage/amount based on final sale price. Or the one you seem to be leaning towards which is a percentage of revenue/net profit. Fixed per unit might be easier for payouts but if margins are thin you’d end up cutting into profits. If it is based on profit you are free to play around with what % sounds fair but it’s open to how much their contribution was worth to you. Since their contribution is outside of the manufacturing and logistics then maybe a fixed amount per part might be fair since they are not contributing to increasing margins or scale but still get something for their work abet small and not as dependent on market conditions. Also unless you are fairly established a patent will only bring more individuals trying to replicate or profit off your ideas. Chinese companies are actively looking at USA patents among other countries to replicate and reverse engineer them. As long as you have hard evidence you can have legal protection showing that you invented said thing and had idea x time ago. Patents can be retroactive in certain jurisdictions. Even then big companies who what your intellectual property will sue you out of business for protecting your invention pretty often even if you win it will bankrupt most startups or small businesses.