I have two suggestions:
1. Show a bunch of prominent examples of star maps on giftable items like mugs and shirts memorializing various events like a birth or a wedding. The more explicit you are about the stars matching the night sky on the day and place of the event, the better. I could see these making for cool party favors, but I think the concept will go over a lot of people's heads (pun intended) unless it's spelled out for them.
2. Offer a la carte pricing. The couple planning their wedding on a shoestring budget might buy a star map for $40, but starting and cancelling a $40 subscription could be too much of a hassle.
Longer term, I might look into partnering with a company like zazzle.com and integrating into their create-your-own-shirt-or-mug-or-whatever pipeline for a royalty.