To purchase a license for an older version of CCC (that is still supported), you simply purchase a license for the current version of CCC. CCC v5 and v6, for example, will accept a CCC v7 license.
Selling multiple versions of CCC licenses has the potential of creating too much confusion at checkout. To avoid this confusion altogether, we've simplified the licensing process such that older versions of CCC can use a license for a newer version of CCC. This not only simplifies the purchasing process, it makes it simpler for you to use that same CCC license on all of the Macs in your household, even if they're running different versions of macOS and CCC. Additionally, if your Mac is running an older version of macOS that requires an older version of CCC, you can continue to use the same CCC license with CCC v7 when you upgrade your Mac or your Mac's OS in the future.