Decoration is two separate problems joined at the substrate
The corporate merch industry groups decoration methods into apparel, hard goods and finishing, but the actual division runs deeper than that. Soft substrates take ink, dye, transfer or thread. Hard substrates take engraving, debossing, etching, pad printing or laser marking. A single brand order across a polo, a cap, a drinkware item and a metal pen often requires three or four of these methods at once, and the supplier's job is to choose the right one per substrate, not to apply one method everywhere. A method that works on cotton often will not work on metal, and a method that works on metal often will not work on poly-blend.
The substrate decides the method before the artwork does. A 100% cotton polo supports screen printing, direct-to-garment printing, embroidery and heat transfer. A 100% polyester polo supports dye sublimation and most heat transfers but is the wrong substrate for direct water-based pigment printing. A structured cap supports embroidery, woven labels, leather patches and screen printing but not dye sublimation. A metal pen supports laser engraving, pad printing and screen printing on a coated band but not embroidery. The substrate conversation comes first.
The artwork decides the method second. A flat, vector, two-colour mark on cotton is screen printing territory, because each colour is one screen and the result is sharp, flat and even. A photographic or gradient mark on cotton is direct-to-garment territory, because a halftone approximation of continuous tone loses the photograph's softness. A complex mark with fine type on a structured cap is embroidery territory, because embroidery reads as premium and survives a long wash cycle.
The run length decides the method third. A 25-piece order of a flat mark on cotton tees is most cheaply decorated by direct-to-garment, because screen-print setup is amortised over too small a base. A 2 500-piece order of the same flat mark is most cheaply decorated by screen print, because the per-colour setup is recovered and the per-unit ink cost is much lower. The crossover between methods moves with the artwork, and a buyer should expect the supplier to ask about the artwork and the quantity before recommending one.
Cost is the consequence, not the driver. Two methods produce different costs because they produce different outcomes, not because one is intrinsically cheaper than the other. The cheapest method against an artwork at a quantity is the method that reproduces the artwork correctly at that quantity, no cheaper.