SMS credits never expire. This means that you can use them up over the time frame of your choice, and spread them across as many newsletters as you like.
When it comes to email credits, you can choose either the Prepaid- or Subscription tariff.
The functions are very similar to those of the SMS tariffs. With the Prepaid tariff you buy one package, with which you can use up your credits by sending newsletters over as long a period of time as necessary. You can spread your credits out over as many newsletters as you wish – and there is no basic fee or minimum purchase.
With the subscription tariff you acquire credits for a month. You can send as many emails as you wish within the month until your credits are used up.
If, at the end of the month, you still have credits, they will expire.
The subscription tariff automatically extends itself by a month.
Exact differences between the two tariffs can be found here.