Hi,
I got some time today to debug the issue and went on, thanks to your FTP access. However, The access that you provided didn't had the permission to modify the files, so I couldn't add any trace to the code on your website:
Status: Starting upload of C:\Users\Nicolas\AppData\Local\Temp\fz3temp-8\cart.php
Status: Retrieving directory listing...
Command: ls
Status: Listing directory /administrator/components/com_hikashop/classes/
Command: put "C:\Users\Nicolas\AppData\Local\Temp\fz3temp-8\cart.php" "cart.php"
Error: /administrator/components/com_hikashop/classes/cart.php: open for write: permission denied
Error: File transfer failed
Then I went on to try to reproduce the issue on my end for the past 2 hours but I was unsuccessful. On my end, the total was correct 100% of the time. So I really need to be able to put traces in the code on your website.