DirectLease API which their mobile client uses to display prices. Using this (private) API might not be legal, so keep that in mind.
Download their app on some android device and install/setup mitmproxy.
Because SSL pinning is used in their app we need to bypass that. For this we use Frida.
Install the frida-server on your android and start it. Next we run frida -U --codeshare pcipolloni/universal-android-ssl-pinning-bypass-with-frida -f com.app_it_up.dl_tankservice (read this post for more info)
Now we can see all the app traffic.
We still need to figure out how to calculate the X-Checksum header.
For this we can use dex2jar in combination with bytecode-viewer or JD-GUI.
Search for the X-Checksum string and figure out what their code does to calculate the checksum.