You will probably find that no B2C customer actually cares about the structure of your business, legally. Your website design matters 10,000 times more in terms of trust gained. Your mileage may vary if you sell B2B, particular B2BigFreakingEnterprise.
With regards to liability: your main defense against liability is not your corporate structure or your terms of service. It is not pissing anyone off so bad that they decide to sue you.
You can, if you choose, get an LLC and create a Terms of Service. These are more to allow you to sleep better at night than insurance against catastrophe: no matter what you do right now, if you get sued, life is going to suck.
You want to sell subscriptions? Paypal Website Payments Pro + Spreedly. You can do it in two hours of coding and about two days or so in time it takes Paypal to review your business. I do not recommend doing yearly subscriptions until you have history: doing so increases your risk profile.
Your main regulatory issue for the typical SaaS app is simple: you owe income taxes on it, both to the US and to North Carolina. Luckily, you will have very good records of sales (because you'll be able to grab them from Paypal). If you want to deduct expenses, make sure to record them and keep proof of them (generally, receipts work fine -- for e-receipts, print to PDF and put it in your Dropbox so that you don't lose it).