I want to buy more space for my iPad and iCloud the $9.99 one?

To upgrade your iCloud storage please see Upgrade your iCloud storage plan - Apple Support

Just to be clear: this storage is online on the iCloud server. its purpose is to mirror what's on your device so that any other devices signed into the account can be synchronised with it. By doing this you are not increasing the storage actually on your phone: it's not possible to increase the storage on any iOS device.

These are the support articles to buy iCloud extra storage:--> Upgrade your iCloud storage plan - Apple Support

and/or:--> iCloud storage plans and pricing - Apple Support

Please, do not confuse iCloud Storage with iPad Internal Storage (which cannot be increased). To better understand, see this page:--> What's the difference between device storage and iCloud storage?

Regards

Giulio