'du' counts files so you need the '*' after the directory.

Not true. Under HP-UX, du -s gives the grand total of disk usage for each of the specified name operands.¹ The BSD man page for du also includes the -s flag (aka --summary), so it is not quite the same as --total (which BSD also has, but not HP-UX). The total can be obtained with a perl oneliner: perl -ne '@d=split($_); $t += $d[0]; END{print $t}'

1 - HP-UX 11.11 du man page