First, i'd drop the boxes not due to freeing up my hands for a tactical advantage, but because that mochachino latte had a lot of caffene, and i'm pretty wired.
As soon as thats done, I'd lament the fragile nature of the gifts, and reflect for a moment on how long i'm going to be spending in the customer service lines for a return/exchange.
As soon as I hear "oh my god he's got a gun!" screams, i'd probably check myself to make sure I'm not printing, as I scan 360* to try and find the car that just backfired a moment ago and caused me to drop my packages.
Shortly after noticing that the teenager was shot and presumed killed, my mind would probably remember a quaint Dickens line about decreasing the surplus population, as i moved quickly away from him, lest the blood spatter my khaki's. Its impossible to get blood out, and I'm not going to be able to chop the legs off this pair and wear them as shorts for quite some time.
I'd then think to myself, "Wouldn't I want someone to act if that was my daughter being kidnapped?" as I fumbled with my jacket's zipper, and then tried to get my gun as I searched through layers of clothing piled on so thick, you'd think my older brother's name was Ralphie, who had a fascination with BB guns.
Several seconds later as the car sped away, i'd bask in the admiration of the onlookers, who can't stop gazing admiringly who stood his ground in the parking lot, and didn't give an inch of ground to the murderers & kidnappers.