We have a one-tier subscription where every feature is included, regardless if you use it or not.
You don’t pay less if you don’t use SimBrief. You don’t pay more if you use both the iOS and the Desktop apps. You don’t pay more if you fly internationally and not just in one continent. You don’t pay less if you only have one addon and only download one navigational dataset.
Specifically in your (FSX) case, we also don’t update the core navdata in FSX, but we do for MSFS. We do however support a much longer list of aircraft addons in FSX than in MSFS. Also there is no in-game panel in X-plane for the same reason, that it is not technically possible, at least for now. As you may know, the FSX platform is from 2006. Had there been an integrated browser in there, it would have been equivalent to Internet Explorer 7, lacking all the technical advancements made in the past 17 years, making any modern web apps completely unsupported (including Charts).
We cannot have the exact same offering on every simulation platform since the technical possibilities are different.
This is the same for most other subscription services, you pay one fixed amount for access to the full range of services offered. It isn’t something we will likely change as it just adds complexity and confusion and most likely no happier users.
However, you can take advantage of all the existing Charts apps on web, Desktop, iPad and Android, and connect it to moving maps on your FSX simulator via the Simlink app Ian mentioned. This is often the better experience anyways as the simulator process is not also loaded with external apps running - and also the Charts display is not hogging your screen real estate either. So if you have a spare computer, extra monitor or a tablet, you can get the full Charts experience. The MSFS in-game panel has its primary use case in Virtual Reality, which I assume is not relevant in your case.
Kind regards,
Stephen