In Salestrekker on top of your main subscription there are additional costs. For these features you are going to be billed on top of your subscription, regardless of a plan:
Phone line: $29.99 per rented phone line per month (no minutes limitation)
Partner portal and Phone costs are charged monthly based on number of added partners and rented phone lines.
To learn how to manage number of users, partners and phones checkout this article.
Credits are bought from your Organization settings > Billing Details tab (available only to admin users). You buy credits manually or you can turn automatic recharge.
Costs for specific feature credit packs are:
eSignature 100 package credits for $49.99 (+GST)
ID Smart 20 package credits for $99.99 (+GST)
PPSR 20 package credits for $49.99 (+GST)
SMS 1000 package credits for $49.99 (+GST)
WebloanQ one month subscription for $79.99 (+GST)
To learn more about payment and additional options, see these articles.