Hi Lexa !
According to Pix4D calculation method for volume error, you should be able to really easily calculate your own volume error using:
Volume error = Area surface x GSD x 1.5
Indeed it could be nice if something might be directly add in "Measure Shape" panel.
By the way, it could be also interesting to add other base plane. Indeed, we are able to do what we want with "flat" base plane using "Mean level" and "Custom level". But when it goes to not flat area, there is only "Best fit plane" available.
I could firstly imagine something like "Custom level best fit plane", where it could be possible to verticaly translate the best fit plane, for example to make it passing by the lowest point.
Regards