If you just need a one off sample calculation, you can pay as you go and just
purchase a single one for <insert price>. If you have several reviews booked in,
or are anticipating that you will need to use the sample calculator several more
times in the near future, you can buy a discounted bundle of 5 calculations for
<inserts price>. After you have purchased them, they will stay as credits on
your account, and when you go to pay for the next calculation, you can simply
use an existing credit instead.