They'll add it when the market demands it.  There's nothing in it for them right now.  It's too soon, and your question is therefore rhetorical.

IMHO, this is the wrong horse to chase right now, but thank you for your persistence and attention.  Build more market momentum for Nxt FIRST, and then ZipZap and GoCoin will see value and fight to add integration.  For example, support Damelon's open-source payment module project.  Once it's finished and a market is created for payment processing, go back to them and do one of two things: offer to help them integrate that system into theirs, OR suggest that they should jump in to become a market leader.  IN the meantime, keep in touch and update them on the growth of Nxt.  Build the RELATIONSHIP.  You can leverage it later.