I know this thread is a bit old, but since it's come back up I'll put in my two cents. Just like I started brewing with an extract kit, I would almost certainly have started soaping with a prepared kit if I'd known such were available.
I see from your HB site that your beer kits allow choice of yeast - dry or liquid - and I think a similar setup with 2 or 3 choices each of typical, lower-priced FO's and EO's that you know play nice would be a great option. (Along with an option to chose no fragrance) Obviously a kit with an EO would be more expensive, much in the way a brewing kit with liquid yeast would be more expensive. Also, a nice optional addition (the same way you handle clarifiers on the site) would be a packet of a single color that complimented your fragrance choices.
I love the idea of the pre-measured and -mixed oils. It solves all the problems of sourcing unusual oils, having left over amounts, etc. I would offer a tallow/lard version and an alternative Vegan version as well.
The best thing, though, is the pre-measured lye. Many people just starting out are a bit fearful of the lye, and it can be hard to find. Then, if you can find it at a hardware store or similar you'll likely have 2/3 of the container left after making soap. Not to mention that a lot of people don't have decent scales if they're not already into a hobby that requires them. One thing to consider, though, is that having the lye pre-measured has the potential to encourage new soapers to add water to lye instead of the other way around - which obviously should be avoided. Being new and not knowing any better, it can be a natural thing to do. I would do everything in my power to combat this possibility. If it were me, I'd ship the lye in a zip top bag with a big warning sticker. It may feel like a less secure method of shipping the lye, but nobody is going to be inclined to pour water into a bag.
I would also consider including a small bag of sodium citrate, sugar and salt to be added to the lye water. The majority of people are going to have at least somewhat hard water, and the chelator will improve their final product. And the salt and sugar are all-but free, so why not include them as well. I know that some would consider this an unnecessary complication and expense, but we're only talking about maybe 3/4 oz of stuff here, and most of that is sugar and salt. I think you're pursuing two customers a kit: 1) brand-new people who want to try a new hobby and will relatively quickly move into putting together their own recipes (like the folks who quickly move to AG brewing), and 2) people who want to soap just a few times a year and will continue buying kits to avoid having to accumulate the equipment and knowledge to do it "the hard way." Both of these customers are served and kept coming back by your kit making the best soap that's realistically possible. (By this same logic, I'd make sure my oil mix included a bit of castor, too.)
The mold is harder. I love the idea of including a cardboard mold, but I work with corrugated suppliers as part of my day job, and I think you'll find that adding a die-cut, plastic-lined mold will be a not-insignificant cost add. Off the top of my head, maybe a buck-fifty for a buildable mold in the 2 lb range. It would be a kick-ass option, no doubt, but that's a big extra cost.