I have a client who already has a merchant account for their physical store.
What specific information about the merchant account will my client need to get from his bank to link it up to an online payment gateway that we will be using for the store?

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-"my merchant account" changed to "the merchant account"

-It is noted that my client will need to get a "high-risk" merchant account separate from his existing physical stores merchant account.

-Though the question is essentially answered, for clarification:

We plan to minimize our PCI compliance needs by utilizing something like Braintree's Transparent Redirect or the CRESecure iForm. So the gateway we would need to give the merchant account info to would be either of those two. Though, the CRESecure method would have yet one more step added into the mix since they would forward data to another payment provider.

It is now assumed that the best place to field this question is to the support team at Braintree or CRESecure.