Yes, the $15 is just for the plants, which is the way most people list plants for sale. The boxes of fresh peppers are priced including shipping.
The bigger plants will vary a lot in shipping costs, even in the same box and the same weight, so I deal with shopping on an individual basis. I shipped 2 nice plants down to Ft. Lauderdale for $8 priority mail. The same exact thing cost $14 to Indiana. The father west you go from here, it will even cost more. The way I ship them, they hold up pretty good if it takes 4-5 days instead of the standard 2-3 days for priority. When people want 4+ plants and if they are going to Texas for example, then UPS ground starts being cheaper. I've been shipping cycad plants for probably 25 years so I have already experimented with plant shipping in general. Most people are pretty nice and I don't charge any extra for shipping above the actual postal cost, so everyone so far has been happy to pay for the plants and then pay for the direct cost of shipping afterwards.