Non-recurring engineering (NRE) refers to the one-time cost to research, design, develop and test a new product or product enhancement.
One of the things I truly love to do is design. I made a choice when I started SMT Designs and the rocketry parts that are on the site, to do the majority of custom designs without charging for the service. I feel that most designs could be used in other projects and I will offer those designs for sale to anyone for the manufacturing costs plus a mark up for my rocketry projects and overhead (if you really want a one off type of design, then we could discuss any official extra charges).
I decided to create a SMT Designs NRE Donation "product" so customers could "buy" as many as they wanted to add to their custom order. A customer is not obligated to buy any NREs, but it is left in their court to add as many "Thanks" to their order as they want to.