I'm a little confused by the question. Are you asking whether affirmative consent to commercial emails in an account opening document allows you to avoid CAN-SPAM rules for opt-outs, or do you just want to avoid having to identify the message as an advertisement?
Transactional emails are ok regardless of consent, although you still shouldn't be deceptive or misleading in your header or subject line (not sure why anyone would with a transactional email in the first place), and I'm not aware of an affirmative consent requirement prior to sending the first commercial/marketing communication to a customer. You have to follow all of the CAN-SPAM rules for that communication (and every commercial email), which are helpfully laid out here:
https://www.ftc.gov/business-guidance/resources/can-spam-act-compliance-guide-businessYou have to provide an option to opt-out of further commercial emails, and you have to honor that request promptly if the individual chooses to opt-out. Affirmative consent in an account opening document would, in my opinion, not override the customer opting out of commercial emails. However, I also think it's still a good idea to include such language in your account agreement so that the customer understands that they may receive emails from your institution and the circumstances under which they may receive them.
I also think that Dan is right in that prior affirmative consent
can get you out of having to clearly and conspicuously identify the message as an advertisement, but I would also agree with Randy that using the account agreement to get there likely wouldn't cover you with the exception and would create unnecessary risk. There could be other ways to go about gathering affirmative consent, but are customers offended that you describe an advertisement as an advertisement?