Why you still have to pay for deflect while starcrossing yout unit in heisho and opponent unit outside battlefield

No — you don't pay Deflect in that spot. FAQ #11783 answers this exactly: choosing your own unit at Heisho, Shell of the World for Star-Crossed switches off Deflect even when the other target (the opponent's unit at base) is the one that has it.

That's because Heisho doesn't name which units' Deflect to ignore — it just says you ignore Deflect while paying for spells and abilities choosing something "here." So it works globally for that spell (Rule 764: ignored abilities are treated as inactive for that payment), and the card that qualifies you for the effect doesn't have to be the card with Deflect (FAQ #11769).

The one limit: it only applies while at least one target is actually a unit at Heisho. If neither target ends up at Heisho, you pay Deflect normally on the enemy unit.