Is there a native way to prevent a logged in user from placing a Sales Order/Checking Out on a SuiteCommerce Standard site, but still allowing them to view and pay on invoices? I tried changing a custom customer center role to have "None" as the Sales Order permission, but it seems to still let orders go through.

Yeah I’m not sure that there is a permissions check for that; also it would be a bad experience to let a customer get all the way to the end of checkout only to see a ‘not allowed’ error. In these situations, I would generally recommend using personalized catalog views. You would create a customer segment of the people you don’t want to be able to buy stuff, and then add them to the unmapped items item collection with the permission most appropriate (eg show products but prevent add to cart). This would stop them much further up the line (ie being able to add items to the cart).

That sounds like a great solution, thanks Steve!