Yeah, its the bundle you can only get from the AMD Never Settle Space voucher.
You get the full game, beta access, and the Arena Commander pass (aka Dogfighting module), as well as the Mustang Omega in AMD colours, which you'll see in your hangar and eventually once completed you'll be able to fly it too. Till then theres a substitute 300i you can play Arena Commander with (a in-fiction simulator, combat vs AI or PVP, and a you can do racing too).
'Backers' are people who've pledged any amount of money towards the project, the cheapest thing being a $5 skin (you can earn everything in game too) and while it isnt a necessity to do this, there are perks which are applied to your account.
Typically they're novelty items, but in the last month there was $10 worth of in-game credits (UEC - United Empire Credits... i think) which at the moment lets you buy in-game posters, weapons, a fish tank, and fish to go in it. At the $42m mark we all got a type of character armor for the FPS combat which is WIP currently.
For this week, CIG have allowed all backers access to the M50 racing ship, so you can go into the game, enter the virtual simulation, and opt to fly that as well as any flyable ships you own (Mustang Omega isnt one of those, so you get access to a replacement of similar value, the 300i, a very nice ship too). So currently, if you've never actually pledged/backed with your own money, you're not getting access to the M50.
A few weeks back they opened up access to all 5 ships (Aurora, 300i, Hornet, M50 & 350R) for the week, and they'll do things like this from time to time to let everyone try the different ships, get an understanding, IMO its useful fo players because not only is it nice, but it means everyone can get a better understanding of another ship, if you keep getting killed by a ship and assume its OP and needs fixing, maybe when you fly it you realise actually it takes a lot of skill to be good with it etc. You gain perspective, which you cant do if you otherwise dont own it.
The Mustang isnt considering you a backer because you didnt pledge/back the game, it works the same way if you create an account and a friend buys you a ship and 'gifts' it to you, you've not actually paid anything so you're just viewed as a member, but a $5 skin would fix that, or anything else really, thats just the cheapest and most common solution.
Finally, melting...
When you buy a ship, say a $110 Hornet, if later on you decide theres another ship you prefer, they release something which sounds much cooler or more to the style you fancy playing, then you simply 'melt' the ship and you have $110 in credit which allows you to put those store credits towards another ship. You could buy a $55 300i and try it, then melt it and buy something else etc.
It means you're not committed to a ship, its a commitment to the games development, and if another ship comes along which you want, theres no 'tough, you bought that one, no swapsies' etc. You simply melt, get your full $$ value back as RSI Store credits, and buy something else. Heck, buy a T-shirt if you want. You cant 'gift' something which has been melted down though, but you can melt & buy as many times as you like really.
I added a chunk of money in there a few months back with 2 unreleased ships in mind, and until they're available (neither are, 4-5mo on) and ive bought & melted a handful of ships since then, and i've now spent the money on 2 different ships i plan to keep.
Hope that explains things, if not, just ask away.
edit - oh, i should also point out that the Mustang, while being valued at $60, if you went to the option to melt it, it'll tell you it'll melt to $0, and ask for confirmation. Other ships you'd get what you paid in, however with this being a promotional thing (you can buy the SC codes for $30 from people, not direct from SC/RSI/CIG) they're not letting you melt it and have $60 credit basically. However, and i mentioned it in the previous post, there is the option to 'CCU' which lets you upgrade your ship to any equal or more expensive ship, for the difference in price. So your $45 ship ($15 = full game package stuff) could be CCU'd to a $55 300i for $10 in theory (that 300i + game package would be meltable for $0+$10, exactly what you've paid basically). You just wont be able to melt it and transform it into its value in RSI store credits.