Free shipping thresholds ("spend $35 more for free shipping") are set deliberately, they're calculated to be just low enough that adding one more item feels reasonable, but high enough that the extra amount usually exceeds what shipping would have cost you.
As an example: if actual shipping would have cost $6, and the threshold prompt asks for $12 more in cart value to unlock free shipping, the retailer profits even after 'giving away' the shipping, because you added $12 in merchandise to avoid a $6 fee.
The math only works in your favor if the extra item you add is something you were already going to buy soon anyway. Adding a random item purely to cross the threshold usually costs more than the shipping fee would have.
A better habit: keep a small running list of things you actually need from a given store. When a free-shipping prompt appears, check the list first, if something on it fits under the threshold gap, add that instead of browsing for filler.
If nothing on your list fits, it's usually cheaper to just pay the shipping fee than to buy something to avoid it, do the quick math before assuming 'free' shipping is actually the cheaper outcome.