As I mentioned in my post I contacted Groundlink after I posted about the issue I found with using their fixed price voucher from JFK.
I received a reply from one of their team, informing me that I was at fault as the address I had entered was incomplete. This is possible, although I cut and pasted it from the Andaz Wall Street web site.
Separately, another member of the Groundlink team posted a comment against the original blog post offering to make things right for people, and saying that there was an exclusion missed off the Jetsetter promo details – namely trips below Houston attract an additional charge.
Anyone having the same problem as myself is urged to contact them to fix things.
I’ve done this and will update with the outcome.