FastMail.FM charges for two main things:
FastMail.FM 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 'Options' screen and selecting 'Upgrade'. See the payment methods page for details on alternative payment methods.
You have a 'FastMail.FM 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.
For instance, if you decide you need an extra 100MB of storage, you can pay US$24.95 from this pool to buy the extra 100MB. If you need more bandwidth than your monthly allowance, you'll be able to buy bandwidth from your pool. The cost of bandwidth depends on your service level (see above). See also Email Resource Limits for details on bandwidth usage.
The reason we're going to do it this way is that it would be completely uneconomic 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.