Best Method to offer options to upgrade mailbox capacity

[HSc]

We offer Email with fixed number of mailboxes, each mailbox with a fixed amount of storage. We also offer ability to add mailboxes 

Some clients buy additional storage space per mailbox. What is the best way to set this up? Configurable Option? Addon? Another way?

I have tried both methods, always using hidden option as it's not necessary to show it to the client. Neither seems to work properly for us so perhaps I'm doing it wrong?

Features we require include:

- prorate (is that correct term) to link fee and renewal in line with existing main product renewal. And only charged for time remaining until anniversary (yearly). 
- text field to enter mailbox address (customer may need to buy separate upgrades per mailbox address
- we offer scaled upgrade (1gb, 2gb, 5gb, etc.)  so the client may upgrade further in the future and/or another mailbox needs a different upgrade (hence need for mailbox field) 
- recurring billing

We are not using cPanel or other "off the shelf" control panel, if that matters. 

Any suggestions would be appreciated.

Thanks for looking.

HSc