What is the difference between 星心相印 and 心心相印？
There is no such phrase in Chinese as "星心相印"。

However, as the comment says, there is a song called that and it is definitely a romantic one. It expressed an admiration to the 'one' who gives the singer great comfort and warm feelings.

Besides, the gift is a "star"fish right?  "star" = "星", and the shells may look alike with the shape of heart. And I wonder if there is any "星" in any of your names.

My point of view, if it is a hand made (or wrote) by herself, it is 100% percent romantic, you have my words. Otherwise if it is only some cheap product(no offense) from a workshop of god-knows-where, than you have to consider if you had misread the signal~