You're the customer, but you're a business customer, not a consumer customer. B2C vs B2B is different, and the contracts involved are different, and it's different way of thinking. The Internet I have at my house prohibits reselling because I'm buying it as a consumer. If I'm buying a business Comcast account, there's an expectation by Comcast that I'm going to be reselling the Internet access (like if I own a coffee shop or something). Thus, imo it's not common, but it's also not prohibited. (But I am not a lawyer and this is not legal advice.)

As for having two integrations, what's your opportunity cost? You'll want a backup integration, but imo that's in the same category as having a backup cloud to run on in case AWS goes down. Which, you do, but the time spent working on that is time spent not working on the product.