How to run or execute the ebook-convert script from a flatpak install of calibre?

According to Debugging section of Flatpak guide and man flatpak-run you can use special --command argument.

So we need to enter the shell by

flatpak --command="sh" run com.calibre_ebook.calibre

and then run conversion here:

[📦 com.calibre_ebook.calibre ~]$ ebook-convert in.epub out.fb2
1% Converting input to HTML...
InputFormatPlugin: EPUB Input running
on /home/user/in.epub
...
Converting XHTML to FB2 markup...
FB2 output written to /home/user/out.fb2
Output saved to   /home/user/out.fb2
[📦 com.calibre_ebook.calibre ~]$ exit
$ ls in.epub out.fb2
in.epub  out.fb2

Or more straight-forward:

flatpak --command="sh" run com.calibre_ebook.calibre \
-c "ebook-convert ~/in.epub ~/out.fb2"

I don't know how flatpak works, but I was able to find the script like this:

$ which calibre
/usr/bin/calibre
$ ls -artl /usr/bin/calibre
lrwxrwxrwx 1 root root 20 Jul  2 20:18 /usr/bin/calibre -> /opt/calibre/calibre
$ cd /opt/calibre
ls -artl ebook-convert
-rwxr-xr-x 1 root root 6400 Jun 25 14:54 ebook-convert