Hi sichtbar-plus
It would depend on how you are thinking to handle this.
You could buy as many add-ons and licenses as you need and handle them all in your single account and, later, you can charge your clients periodically for these licenses but, in this case, you'll be the licenses' owner and all our invoices will have your company's name.
But if you prefer your clients to own their own licenses instead, so they pay their subscriptions to us and have their corresponding invoices, I'd recommend you ask your client to buy the add-ons directly instead (with their own account) and just giving you the license key is enough for you to set up the automatic updates on their behalf.
Please, take into account that if you buy any add-on using your own account, it's not possible to assign them later to a new user because this new user wouldn't have a subscription created and we can't do so manually. So, at the end, your client would have to purchase the add-on again .
I hope this helps.
Best Regards,
Salva.