Hello,
I'm sorry but I don't understand your issue and I cannot reproduce your issue.
Your product do not have any stock limitation (there is no quantity stock available in the input) and you have set-up the Cart Notification plugin to display cart messages.
When I enter a value, I right got the cart notification and if I enter a really really big number, I got a warning telling me that the quantity value is too much.
For the test cases ; I'm not redirected to the cart.
I don't know what is your "instock" and what you have configured for your plugin. But the cart notification system looks to work fine in your website.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.