In Safari Browser I have the problem that change quantity of added products are not changed properly. In Firefox and Chrome it works as expected. There are some Apps installed but due to the fact, that it works without problems on other browsers could be a bug?
You can try out via https://saint-cosmetic.com and see the attached video. Please support, thanks!
Works just well for me on Sierra, Safari Version 10.1 (12603.1.30.0.34). What are your system specs?