For this Kickstarter, I'm afraid we can't do add-ons--if you want a particular item, you have to pledge to the tier that contains it. We tried to keep the prices reasonable (the very highest tier is $100) and NONE of the tiers can sell out, so even if the item is limited edition, we'll print as many as the Kickstarter needs. <3
The paperbacks and some of the merch will be available in the Sparkler Shop/Hivemill after the Kickstarter is fulfilled, but for anything limited edition - the smutty doujinshi and the signed bookplate - we can't guarantee there will many (or any) copies after the Kickstarter is over, since those are special items we're printing up for this Kickstarter. So if you want them, the only guaranteed way is to pledge!