slash at the end of the url address

-- url of the page with the problem -- : www.autorelaks.pl/shop/
-- HikaShop version -- : 2.6.3
-- Joomla version -- : 3.6.2
-- PHP version -- : 7
-- Browser(s) name and version -- : latest chrome, firefox
-- Error-message(debug-mod must be tuned on) -- : The cart is empty

Hi,
I noticed some time ago, that if I add slash at the end of the url address of my website in my .htaccess so google is happy and this speed up a bit url loading I can not add products in hikashop.
for example
www.autorelaks.pl/shop/ or www.autorelaks.pl/shop/product/5-leu/ or anything with / at the end of the url, Hikashop will not add new product it will just say The cart is empty.
If I remove slash from the url eveything is ok.
To add slash to all my urls I use this in .htaccess
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^(.*)([^/])$ /$1$2/ [L,R=301]
I tried other rules but with the same problem.

I really would like to have / in all my urls . I search the topics but I could not find answers. Please can you help me.

Thanks
Adam