What To Spend Your Credits On
Alter is free to play, and this makes it openly available to anyone with an Internet connection. You don't need to pay anything to have a great gaming experience, but there are options for those who can and are willing to spend real money. There are a lot of things you can buy for real money, but it can be difficult to know what is useful and what isn't. Some things will make your gaming experience involve much less hassle, some are just there for cosmetic reasons, and some seem useful, but are fairly useless.
Weightless Containers
There is nothing more important than a weightless container in Alter. If you were planning on spending any real money on the game, this is the first thing you should consider purchasing. It is not required to play, but it makes inventory and weight management so much easier. I have played both with and without a weightless, and it's a night and day difference.
The non sale price for a weightless is 2500 credits; This translates to $25.00 USD (1 credit is 1 cent). The default capacity of a weightless is 500 pounds, and this should be more than enough. It works as any normal bag, just a bag which has no weight no matter how much stuff you put inside of it. You can put up to 500 pounds of weight into the weightless, and it will be free weight, it does not count as weight in your inventory. This means you won't lose movement faster, you can carry more equipment sets, you can use consumables to stay running longer, and it lasts forever. You can even increase the weightless in small increments as you progress in the game if you find it necessary. It is 100 credits for 20 more pounds, and this works out to the price of a weightless if you were to do the math for a 500 pound expansion.
As far as I know, every person who has been playing alter seriously for a good amount of time has a weightless, or is planning on getting one. It is the most commonly seen credit item you will se in game...by a long shot. There is a reason they are so popular, and it is because they are so useful. If you are a necro you can store all your souls for 0 weight cost. If you are a warrior or thief you can hold all your different weapons for 0 weight cost. The same goes for druid and sun and star catchers, and for whatever mage's and clerics's need to hump around. If you were planning on getting credits, a weightless is always my suggestion as the very first thing you should spend real money on.
Storage Lockers
The Second most useful purchase you can make for real money is a storage space. These are located and accessed at any waypoint, and it holds 1,000 items by default. The size limit can be increased, but in my experience it is not needed. There is no weight limit, only an item count limit, and you can put items in and take items out universally at any waypoint in game. This means you can put something in at Seaside, then take that item out from the Dragontooth waypoint, or any other waypoint. This is exceptionally useful for keeping down on inventory clutter, and also for saving items for when your character levels up and progresses in game.
One of the best things about a storage locker is the free one month trial you get with a new character. You can activate it with the command 'storage activate'. This can only be done once, but once it is done you get a free month of storage time for 1,000 items. After the free time expires then it runs about 2,000 credits for a years worth of time, but if you pay attention to sales you can get it for much cheaper. There are commonly 20% or 30% extra sales on storage time, and taking advantage of these will get you a lot more time for less credits.
I'm sure there are people who play without storage, but I haven't met any of them yet. I can't imagine trying to play without a storage, but it is possible. This just makes it easier to hold onto resources and equipment for higher levels so you already have everything needed when leveling. One of the daily rewards is also two weeks of storage time, so if you are lucky you can get out ahead of the curve with a build up of free time.
Death Free Cards
This one actually isn't really necessary, or needed in any way, but it can be nice for new players when they get into higher level areas. The mainland can seem a bit daunting after the hand holding of the first three islands, and this causes more than one player to drag their feet in pushing forward. This can be fixed by buying a death free card. With a DFC you don't have to be afraid of exploring, and seeing what is out there on the mainland. Getting in over your head becomes a learning experience, but one where you don't lose gold, experience, and spell or skill knowledge.
The death free cards aren't expensive, only 200 credits ($2.00). This is cheap enough to pick one up, but not something you want to be relying on for your entire game life span as that would get expensive. The key is to use a DFC to get past the hump from the third island to the mainland, not to use it as a crutch to never get better at the game. If you can manage this, then you'll be running around the mainland pretty quick, and seeing how much the game has to offer.
Conclusion
Alter is free to play yes, but you can make some things easier by spending a few dollars on the game. Not only do you get something useful for the money, you are also supporting a game which has been entertaining hundreds or thousands of people for over 25 years. There are those who spend real money for a pay to win mechanic, but I would never suggest or tell anyone to do that. Quite the opposite, I judge these people, and will never view them the same as someone who has learned how to play the game without sucking. Buying a weightless and storage time is not the same as buying gold with real money and relying on nothing but consumables to stay alive. You can choose how you want to play, just get a weightless and some storage time first, you can thank me later.