Do you need more than your weekly fair-use credit allowance? You can purchase extra credits. These extra credits will only be used when your weekly allowance is exhausted.
For reference, the weekly fair-use allowance is 10,000 credits. This means that by purchasing 10,000 extra credits, you double your weekly usage limit.
Extra credits do not expire. They are valid for as long as Sigmund exists and uses the current system of fair use in combination with extra credit.