The only existing cost is the handling fees for the gift list / registry. It is linked to payments made by guests, these costs can be edited so that the guests or the couple pay this fee. In the case that the guests asorb these fees, there will be no costs at all for the couple. 

How do I do it?

In your Zankyou, under 'Gift List / Registry' < settings.

You can change these settings whenever you want.