I think it will depend on the advertiser so probably best to add to cart after clicking the link. I would have guessed most integrations send the click identifier along with the transaction on checkout. However I just read the following from amazon, which makes it seem like the click id is associated with the product as it's added to the cart in the 24 hours after the click. At least for amazon, it seems that as long as it's associated to a click you have 90 days to checkout and pay for the product. If this is the case I'm hoping it's possible to have products in the cart associated to different click ids.

https://affiliate-program.amazon.com...er%2090%20days).