If You want to update from Quick.Cart v5.0. Check changes using Total Commander. Select file in Quick.Cart v5.0 (first column) and the same file in Quick.Cart v5.1 (second column) and click in menu "Files" > "Compare by Content". You will find changes made by us.
when the system generates a link such as http://www.yourcart.com/?3,1111 (which should display product #3 with name "1111") then the 2nd number is considered to be a page number 1111 and the 1st number is considered to be a name of the page, name "3". So it actually wants to display a page 1111 and if there is no such page in the system, it says "cannot be found".
Can this behavior be fixed for people who use only numeric product IDs for product names?