[inebriate]

its not product specific, so if you want to put it in products.tpl you can but it wont be on a per product basis, so you would put it before the {section}{/section} (or maybe it was {foreach}{/foreach}) loop or after...i would suggest putting it in the minicart, somewhere visible to the customer that wont get mixed up with other info

an item added to cart will imply that any item was added, so putting it next to a single product in a products list is not a good idea, which is why i suggested putting it before the loop

you can make it product specific but that requires more code since you need to check not only for the product but match the product options as well

this code was made for 4.1.x, 4.0.x should be similar but i havent bothered to look yet

what this code does is it will display the message for 5 seconds once the script finishes loading (not to be mistaken for a complete page load) and then clears it, you can adjust the time if you so choose