SIP Inspector Pro licenses are paid through PayPal or with a credit card. When paying a license fee make sure to include Machine-ID for which you are buying a license for.
When making payment select the appropriate option and do no forget to include Machine-ID in its designated field.
If you are buying a license for business purposes feel free to ask for the official invoice.
NOTE: License is tied to a physical machine. It is not possible to move the same license between different machines!