One-Time Invoicing for Consulting

I do consulting work in addition to providing subscription services, and would like to bring this aspect of my business under the M2 roof. In this scenario, I have a customer who never needs access to my subscription products, I just want to provide an invoice on my site which they can pay via PayPal (also via my web site). My site is setup to allow only a single subscription per user, but they may have need for consulting in addition to a subscription service.

I've started on this a bit by setting up a "Consulting" membership with "One payment for permanent access" selected, but they should never have any access to anything beyond their account page. I considered using finite access, but the minimum I can place on that is one day, and I worry about these customers gaining access to things they shouldn't either due to bugs or misconfiguration.

What would be the best way to implement this sort of configuration beyond manually deactivating their "subscription" after successful payment has been made? How, using the single subscription limitations, can I also offer things to subscription customers like additional licensing seats or enhanced paid support hours without causing issues with their current subscription levels?