Best to just avoid the Marketplace, especially if you are on Steam.
Asobo/Microsoft don’t and won’t provide any proof of purchase (which you will require in the event of something going wrong and a dispute with support)
This is not a bug, this is by design (zendesk support official response to my own dispute regarding this)
The Steam receipt does exist, but this is not itemized. It does not identify the module that you have purchased only that you have purchased “credits” for use for “in-game purchases” - Steam support are unable to assist with any dispute that happens after the purchase of these credits.
Microsoft also require “validation” of these modules every time you start the game in order for them to be available to you. - which explains why on occasion a user may discover that their add-on module is not in game. (Zendesk support official)
After the recent global Xbox validation server outages this really doesn’t inspire confidence.
Contrast this to the third party vendors from whom you can buy these same aircraft/sceneries etc.
You receive a proof of ownership receipt
The ability to re-download the module and often any developer updates are available sooner through their services (and in the case of the one I use most frequently there is an email notification that an update is ready for download.)
Having made that download you can store a local backup.
No validation issue from Xbox will make your purchase evaporate or be unusable.
It’s a no brainer really.