Assuming coffee is Kosher, would it be alright if my non-Jewish co-worker brings me coffee on a trip to the corner cafe? May I assume that the co-worker indeed purchased a Kosher item and has not adulterated it in any way?
Foods sent via a non-Jewish courier and not sealed are forbidden to the (Jewish) recipient, as a precaution in case the courier switched the kosher food sent with non-kosher food he picked up along the way. However, most pos'kim hold that where there is no incentive for the courier to switch the food, there is no concern; moreover, if there is no opportunity for him to do so, there is no concern. So if there is a cheaper non-kosher source of coffee in the neighborhood (incentive and opportunity to switch), the route the courier is taking is not heavily populated (opportunity to switch, as no one will see him do so and report it to you), and the lid is not tamper-evident or is easily replaced with another (no seal), then the coffee would, it seems to me, be forbidden to you; otherwise, it seems to me the coffee would be permitted to you.
But consult your rabbi for practical halacha.
Source: Halachically Speaking, volume 6, issue 6 (which cites further sources).