The Discovery Bible Project does not calculate or collect tax. We use a service called 'FastSpring' (FS) who collects any sales tax payment that would be due for your place of residence. Some places have no sales tax. You will be able to see whether you are required to pay sales tax in your location when you click the purchase link to the checkout page before payment.
We would not be able to sell Discovery Bible Software (DBS) worldwide without FS because we don't have the resources to pay for accountants to work out the sales tax. FS ensures we can provide DBS worldwide and remain tax compliant - the specifics of how they do that are out of our control.
If you have further questions regarding tax payments, we recommend contacting support@fastspring.com.