Do I need to incorporate?
Assuming you're in the U.S., incorporation is neither here nor there in terms of transactions and taking in income.
Company payments to sole proprietorships and to partnerships need to be tracked by companies, and they are required to issue IRS form 1099 payment statements to non-corporate payees annually, when payments are above $600, and some companies elect to issue them to all non-corporate entities even if less than $600.
Another commenter mentions form W-9. This is merely a request to the IRS for a Tax ID number for some business. You can see that on the form, all of the types of entity you could be are mere check-off boxes. You can have several Tax ID numbers, as an individual running several separate non-corporation (that is sole-proprietorship) businesses. There is nothing special about a corporate form for obtaining one or more Tax ID Numbers, also called EIN, Employer Identification Number. The businesses that pay you want your EIN in order to issue a year-end 1099.
Corporate-ness typically involves several values (and administrative costs) to a business. The leading one, is a limitation of liability, when things go wrong, rather than your own assets (however small or large your own personal assets may be): it is the corporation that is acting, not you, and the corporations's assets are looked to remedy errors and injuries. Similarly, for partnerships, all partners are liable for actions of their partner: this is what makes a corporate form useful for a multi-investor, multi-partner business. Sometimes insurance is available, and not so expensive that it can be an adequate means to meet the some potential liabilities, sometimes not.
Then there are tax and income issues that are different than the sole-proprietorship.
The questions you should ask your own accountant, and lawyer, and business advisor, which you now are on notice that you should have:
1. What particular advantages and costs come with the several typical operating entities you could use: Corporation, Limited Liability Company (LLC), and sole proprietorship (and partnership if you have a partner).
2. Separate from that, what are the tax consequences of each entity, and several Federal tax-filing-statuses: sole proprietorship, partnershp, LLC elections, Subchapter-S, Subchapter-C taxation (LLC and corporations)?
3. Is there any investor and partner involved, and can a non-partnership entity aid in structuring operations, and limiting your liability from your partner's incorrect actions?
That said, you should probably incorporate. Incorporation requires very little time and almost no money, and it provides varying measures of liability protection, anonymity, professionalism, legal status, and tax advantages.
Your customers are going to be large companies. They are not going to pay an individual for conducting business transactions. In fact, they will probably need a W-9 [1] from you.
Also, don't forget the liability that you will have if you are not incorporated.
[1] http://www.irs.gov/uac/Form-W-9,-Request-for-Taxpayer-Identi...
That said, I can't imagine a scenario where a "business employee who is entrusted with a company card" would find it acceptable to spend the company's money paying for a personal, non-business service.
FYI - prefix your questions with "Ask HN:" and you'll often get better visibility.
If you receive the payments as an individual (as an American), you'll have them taxed at the normal federal income tax rate, state tax rate, and local tax rate, so you'll have to set aside 50% of your income for taxes.
If you're not sure how successful your venture will be, start off taking payments to yourself. If you grow bigger, then put a company behind it. It's annoying to form a corporation, maintain the paperwork on it, then end up only processing $50/month through it.