Update: Just tried the site again (departing this weekend) and it worked!
Regarding how far in advance one can do it, like others, I found that I could not enter our arrival and departure dates until our trip was less than seven days away, and thus I could not make it through the site to the payment screen. Once I was inside that time frame, however, I could make it to the payment screen but the site wouldn't accept my payment. Retried several times this week, with no success, hoping that the website issue had been fixed. Tried one last time this morning, just for the heck of it, and I was pleasantly surprised.
I hope this means that the website glitch has been fixed and that other travelers will now have success as well.