The download button opens the iTunes App Store, where you
may continue the download process. You must have iTunes installed with an
active iTunes account in order to download and install the software. This
download may not be available in some countries.
From British Forces Post Office:
The BFPO Track & Trace Application will enable certain letters and parcels from carriers such as Royal Mail, Parcelforce and BFPO to be tracked whilst the item is in the BFPO mail pipeline - (see https://www.gov.uk/british-forces-post-office-services for a full list of products supported). By entering or scanning an item's barcode, the BFPO Track & Trace App will show the route that the item will take, the expected delivery times at each node and its actual location; it will also provide a graphical view of that information on a map. The App also supports push notifications so you will receive up to date information of where your item is and its expected delivery date. The BFPO Track & Trace App compliments the BFPO Information App https://itunes.apple.com/gb/app/bfpo/id567839256?mt=8&uo=4 which allows customers to access up to date information on the most popular range of products and services that the BFPO provides; it will also facilitate the printing of a barcode that can be tracked using the BFPO Track & Trace App.