FastMail charges for two main things:
FastMail has a number of service levels. Each level has access to different features, and different limits on monthly bandwidth, disk quota, etc.; see our pricing table for comprehensive details on the personal accounts service levels, including the cost of each level.
You can upgrade your account using a credit card by going to the 'Billing & Plan' screen and selecting 'Upgrade'. See the payment methods page for details on alternative payment methods.
You have a 'FastMail account pool'. All extras will be taken from this pool, e. g. excess bandwidth, SMS messages, extra storage space.
This pool will obviously start off as empty for everyone. If you need to pay for some extras, we get you to authorise a credit card or paypal charge of $10-$50 and put that in your pool. Then as you need the extras, you'll pay with money from your pool.
The reason we do it this way is that it would be completely uneconomical for us to do the micro-charging each month of excess bandwidth, storage, etc. We need to do a minimum of $10 not to get eaten alive by transfer charges.