They already support Lightning.
Right. I'm buying gift cards frequently from that place. 
Lightning transactions only make up around 15% of their business, while zero confirmation transactions make up 60%
Ouch! People, justifiably, don't use lightning often yet. Either because it's responsibly tough to run your own lightning node, or because just using bitcoin without custody is already too confident for the average Joe to make him get involved further. They need to understand they can't have that unconfirmed option forever though. To increase that 15% they could provide greater rewards for purchases with lightning.