In the case of your "kit", very little "skill" is required. To install your "kit", you need to push the brass barb onto the tubing on the Ryker. To install a DIY "kit", you need to do the same, twice. "Skill"?
I am very familiar with producing, marketing, and selling "kits". I have done so for several automotive specialty products, and currently I am selling this one:
https://carveshop.com/electronics/
In my case, it does take electronic assembly skills to build the "kit", that not everyone has. A barb and a hose, not so much.
In the US we do not need to go to the Post Office - we can print postage at home and have the letter carrier pick up packages at our mailbox or door. And the Post Office provides free shipping material. But the point is not the cost of marketing, packaging, shipping, etc. It is all about the cost and complexity of the "fix" itself. Compare the complexity and cost of my electronics "kit" vs your Ryker "kit" to see what I mean.
If you are finding buyers for your "kit", more power to you. All the best.