Many thanks for your reply NJFlye606.
For clarity, I used the Wallet facility on Steam to pay for the enhanced version of the airport.
In case it helps anyone else, the following may also be helpful.
I reached out to Microsoft for help in resolving this at the link that you kindly sent me and quickly received an email back acknowledging receipt and asking me to double check everything asked for, was there. They also asked for “…a screenshot of the Steam Wallet Receipt (.PNG or .JPG format) OR a copy of your bank statement that proves that the funds were withdrawn.” which I sent.
MSFS Support came back to me within a day saying that unfortunately they didn’t have a record of the transaction and offering a refund. They also added that I’d need to copy their email agreeing to one to Steam. They further advised that to prevent the same thing happening again, if using Paypal, to “…add funds to the Steam Wallet 24 hours prior to purchasing from the Marketplace to ensure that the transaction is processed by Paypal and Steam at the time of purchase.”
They then went on to say:
"If you’re paying with a bank card
• Close Microsoft Flight Simulator
• Add Funds to your Steam Wallet with the payment method of your choice
• Wait a few minutes then launch Microsoft Flight Simulator
• Go to the Marketplace, and purchase your content with the Wallet Balance
Alternatively, you can purchase DLCs directly from 3rd party platforms such as Aerosoft or SimMarket. If you do, the packages must be installed in the Community folder."
Hope this helps.