Do I need to set up an account and payment information for each provider I intend to use with libcloud? Are there any service offerings that allow you to sign up for payment once through them and interact with all libcloud listed providers easily?
Yes, you do. libcloud only offers an organized mechanism to interact with the public APIs of various cloud providers - it isn't a business of its own.
I've never heard of any company that lets you manage accounts across cloud service providers like you describe, other than reselling (like Heroku being built on AWS). You'll need to decide on what's valuable to you in a provider and do some research to see which one is the best fit for your needs!