It's unlikely that you need a Merchant Account as an early stage startup. Worry about that when payment processing fees and the time for money to arrive in your account becomes unmanageable with a Gateway like Braintree or Stripe. Even Paypal may be sufficient for you.
You need your EIN before you can open a Small Business Bank account as a Corporation. Until that time, you can open an account as a Sole Prop or even a Partnership.
I know you can use Stripe and Paypal as a Sole Prop and EIN will not be required. I assume Braintree is similar (we use Braintree with our C-Corp and really enjoy working with them).
Just because you can do something, does not mean you should. You're taking on a huge amount of risk to save a minimal amount of time and investment. Michael highlighted some specifics above involving chargebacks, but there are other reasons. I'm not going to go through all the reasons to incorporate, assign inventions and price shares before you take in money, but it's important for you to understand.