I would like to include the link to purchase travel insurance however I can't figure out how close to check-in they are able to buy it. Does it have to be done AT CHECKOUT or can they do it AFTER they pay for their rental?
I think it must be at last a month prior to arrival.
Yep, it's the later of 30 days before arrival or the final payment. In other words, if they book more than 30 days out, they can take the insurance later up to 30 days before arrival. If they book inside the 30 days they can purchase insurance at check out or the same day they book, but not later than that.
The other exclusion on last minute is that the insurance won't cover storms in progress at the time of purchase. If the guest purchases the insurance before the storm is named, that's fine. But after the storm is named it's an excluded item.