The date moves, but the method does not. Fix the third Sunday of June in your calendar, count back three weeks, and Father's Day stops being a scramble.
The third week is for ordering the posted gifts. This is the single most useful thing on the checklist. Ordering mid-November, well ahead of the courier crush, means your gifts travel while the network is quiet and arrive with room to spare. Chocolate keeps, so early delivery is a feature rather than a risk.
The difference is in the contents. Where an egg is a single large piece of moulded chocolate, an adult hamper is a selection: darker, single-origin pieces, hand-finished pralines, perhaps a share bag to keep back. It rewards slower eating across the long weekend rather than one sitting, which is closer to how most adults actually enjoy chocolate.
The second is that a chocolate collection can be pitched to the person rather than the occasion. Someone who prefers darker, single-origin pieces gets exactly that. Someone who likes hand-finished pralines gets those. A collection lets you show that you know the person's taste, which a bunch of roses, however handsome, cannot.
The second week is for deciding format. A boxed assortment for the adults, a share bag chocolates bag for a family, individually wrapped pieces for colleagues and teachers. Settling this now means you order once rather than returning three times. The chocolates are handcrafted in Newbridge, Co. Kildare, where Mary Ann O'Brien started the company in 1992, and the range covers most of a gift list without needing to look elsewhere.
Chocolate works as a thank-you because it is generous without being personal in a way that feels awkward. A teacher does not have to wear it, display it or find room for it. A boxed assortment can be opened at home over the holidays, shared with the staffroom, or kept back as a treat, and it says thank you without asking anything in return.