Do you really need physics in your game ? It seems like a bad question because if you are working on it, the answer is "Yes".
But think about it:
-First a real physic engine is a pain (If you know, remember the time it took to the Blender team to have a decent GameEngine with physics...) and a perfect 3D physic Engine is out of reach. Little Big Planet has a nearly perfect one because it's a 2D engine... And synchronizing all that stuff with server-client model will be a challenge !
-Second, in a cuboid world, is it necessary to have a physic Engine ? A cube location is either free or not, and that's enough ! If you really want a physic engine, the bounding-box of the shape should be enough ! Look at Cube world for example (You see I didn't mention Minecraft (Ooops, I mention it...))

Of course, you have probably some thought for using this physic engine in the game, and that's cool. But maybe you should push this after major GamePlay elements like quests, complex underground, tools, etc...

Thanks for reading, and again it's only my humble opinion. A physic engine won't sell your game but some real gameplay elements will IMO. For example, I bought Miner wars when the first mission prototype was in the game ! At this stage, we can say "Now it's starting to be a game, not only a demo"

Anyway, good luck and congratulation for your work man !

Gfamad

Your approach is interesting and it smell like a "free to play" or "Pay as much as you can" method. The incremental method by 5$ is also interesting because it avoids very low donations (The last Humble Indie Bundle had some 1 cents payments...It's highly disrespectful for the team who made games).

Depending on what will become Mythruna, a medium price around 20$ could be fine for a game with quest, infinite world, cool gfx, etc... But personally, I'm ready to pay more if the game became a serious alternative to Minecraft (even if I know it will become a different game).

But maybe, after, you could sell it with Desura/Gamersgate or ... steam ! Think about 3079 for example...

Good luck with this game Paul !

Gfamad

I do agree, Paypal seems today to be the most common way for paying on Internet. Let's see if this poll tends to prove it 

Gfamad