Sell Invoicing as a stand-alone add-on license
Rather than selling Invoicing as a Business App within the Business Premium subscription, it should be sold as a separate, stand-alone license. In my case, I have an E3 license. So, in order to be able to get Invoicing, I also have to add a Business Premium license to my account. So, I'm essentially paying for all features except Invoicing twice. $32.50/month for an E3 and Business Premium license.
I have the same problem. My company gives technical support and advice to small business. Microsoft Invoicing looks as if it would be a really useful product for some of our clients. But without being able to install and use it ourselves, so that we can build experience to support them, it's not practical to recommend it. As we have the top E3 licence I would have thought that you would have automatically included it.