Hello,
Some observations before answer you, an alert windows isn't the most simple way to inform your customer about, by example from a total cart to gain a free shipping than an alert windows.
Why not, a custom field with a text that will inform your customer in the product page or where you need it ?
For an alert windows, you need a trigger to display this window alert, the question is which trigger ? And where ? In the checkout ? the product page ? etc etc... If we understand we are able to understand the context, our answer will be more detailed.
From this details, our answer will be more precise.
