One of our tenant is low on storage and I know we can add additional storage as an Add-on. Given that the additional storage is not cheap, is there a better way other than to buy whole lot of storage and start paying for that right away? Is there a pay for what you use sort of option?
It really depends on the use case. Another thing to mention is that if you plan on expanding your workforce in the future, every additional SPO license will add to the total storage allocated to your tenant. But there is no "pay as you go" option, and if you want to remove all the purchased add-on space at some point, you will have to contact support.