What ways are there to avoid getting billed more than what you could afford?

There's none that I know of. In my opinion, the best way to make certain you won't get billed more than you can afford, is to ensure you can pay for particular services throughout an entire month, a day, an hour, or however long you're going to use them for. Of course, if you're doing something like automatic creation and deletion of Linode's and other such items, that would be more difficult to accomplish.

The problem with ensuring you're not billed over a particular amount is quite simple. Linode has no credit offering services, and should you go over the amount that you owed Linode, you would be in debt with them, and if you weren't able to make payments, you could have debt collectors coming after you. It's all about proper budgeting to make sure you can afford what you want to get.