I bought it back in March 27th, 2015 (I got the dates wrong, whoops 
), for $34.99 (The game) + $19.99 (Adventurer Edition upgrade) on Steam, kinda sorry I didn’t get it from the playboundless.com site.
You can easily get the explorer edition, as the only difference right now is with the world builder being available for higher tiers. You can also upgrade later on if you wish.
Q: Can I upgrade my Boundless package?
The main difference between instant access and launch key is this. Instant access lets you play right now, launch key will let you play after 1.0 comes out, which has plans to come out this year, but nothing certain yet. Those extra keys, I believe are giftable.
I bought the adventurer pack, and just recently upgraded to wayfarer.