Is there a word for when you go into the supermarket to buy a single item, and you end up buying half a dozen or more things that you didn't realize you needed, or wanted?
Oxford Online defines impulse buying as:
The buying of goods without planning to do so in advance, as a result of a sudden whim or impulse.
: to buy in excess of needs or demand
: to make purchases beyond one's needs or in excess of one's ability to pay
So you could say something like
I ran into the store for a jug of milk, but I totally overbought and ended up with five bags of stuff.
Don't send your father to get the lemons—you know he always overbuys!
Good marketing. They got you to buy stuff you didn't even know you wanted. I don't understand the confusion. Good marketing gets you to purchase items you didn't think you wanted or needed. That is marketings job, to get you to buy and buy and buy. To convince you you must have what they are offering. There is no real word for it other than marketing. Which could also be shopping in some context.