1 free category, pay per extra category

Hi Guys,
I’m wondering if anyone can suggest a workaround to this problem.
Currently you can either:

Have all categories “free”, but include multiple(s) in specific packages.
Or:
Charge per category

I would really like to allow for 1 free, then pay for additional categories as addons to your existing package.

The only way I can figure so far, is to charge for all categories but give a coupon code to everyone to cover the initial cost of the first category – but this gets messy, and involves having a coupon “open” at all times.

Anyone else using this strategy and have any suggestions?

Thanks,
Paul