Hmmm, just found this thread. Been a crazy summer here, so had to step away from hobbies for a bit.
As another hobby business owner I'd like to congratulate you on taking the leap.
As a business owner, and manufacturing engineer, I'd also like to offer some words of wisdom for you as well.
First thing is, never never never never never never (did I say never) stop re-evaluating your business.
Always keep looking at ways to improve your labor efficiency. Always keep looking for ways to reduce your materials cost.
If you do something in your process, always keep a keen eye on how to do it faster/cheaper/easier/more accurately.
In industry terms it's called "Constant Improvement". Meaning you never stop looking for ways to improve your production methodology.
Same goes for your record keeping, sales and marketing, even your designs.
Which leads to.....
Profit margin is king. $5 profit on a $15 kit is 33.3%. Fine if you're only direct selling, but if you ever plan on having re-sellers it's nowhere near enough.
Re-sellers need to make money too, and if they're only making $2 per kit they're gonna pass. Rocket inventory can sit for quite a while, taking up space, taking up money, waiting to sell.
If kits sit too long and go on discount to move the inventory, re-sellers are going to be losing money on every kit they sell, and you're not going to get repeat business.
Also factor in the financial cost of your time. If it takes you an hour in total to get each kit from raw material to packaged and ready to ship...... you're actually not making anything from a profit standpoint.
They're a lot of hidden costs that you're not factoring in yet too. Every business is different, so you need to find those costs and make sure to take them into account. Every penny.
As your business develops, you'll be able to read the pulse of your customer base. Stay on it. You need to keep your customers happy, always.
They may not SAY things are making them un-happy, but you'll know when they are souring to you. Fix it fast, fix it right, fix it loud.
Loyal customers are your bread and butter, they will keep you in business.
Angry customers will group together and breed like an uncontrolled infection if you don't stay on top of it.
Now, as to your most recent question on your kit with larger motors?
Honestly, you need to calculate it, build it, test it, fly it and determine what happens. Not just once, but repeatedly, under the conditions that customers will see.
You can always recommend which motors to fly it, sure, but just about everybody who's flown rockets has gone outside those recommendations.
Always assume somebody is going to do exactly what you don't want them to. Determine the WORST case, and make that as safe as you can.
There's a reason motors go through such a rigorous testing and certification process.
Customers are wonderful people. Some of them are very very stupid.
If they fly it on a C6 and lose it?.... that's rocketry.
If they fly it on a C6, it tumbles, and spears somebody in the chest? .... That's YOUR fault for providing an un-safe product.
Remember, a C6 is a heavier motor, not just longer burning. So it's going to affect your Cg to Cp relationship. You DID calculate that, right?
The NAR safety code and NFPA are a great starting point.