How can one reconcile the permissibility of a Muslim working for a company that is generally acceptable, even though some of its branches might sell alcoholic beverages or engage in unlawful activities—based on the principle, "What is permissible subordinately is not permissible independently"—with the impermissibility of assisting someone in purchasing or repairing a phone, knowing with high probability that they will use it to congratulate Christians on their holidays?

There is no harm in you helping someone who wants to buy a phone or repair it, as long as he intends to use it for permissible things. Even if you know that he might use it to congratulate a non-Muslim on their holiday, the sin for that is on him, not on you. This assistance is not among the forbidden forms of aid because it is indirect and unintentional. The Assembly of Muslim Jurists of America has divided aiding in sin and aggression into four categories: direct and intentional, direct and unintentional, indirect and intentional, and indirect and unintentional. The Assembly has ruled that the first three categories are forbidden, while the fourth category is permissible.