Design readability at conversation distance. A snail birthday gift t-shirt has to land the joke or the species at the same range as a 'happy birthday' across the dinner table; shell spirals, slime trails, and antenna details that disappear at five feet end up reading as a smudge.
Clear niche signal over generic cute-animal. The wearer wants the design to read 'snail person', not 'cartoon mascot'. Helix-shell spirals, kawaii slime-friend faces, scientific-illustration accuracy, or insider phrases like 'Snailed It' signal community membership without explaining the joke.
Style register that matches the birthday person. A goblincore moss-and-leaf aesthetic suits the slow-living fan; a Vitruvian Fibonacci-shell drawing suits the Gastropod Enthusiast or Malacologist; a 'Just A Girl Who Loves Snails' design suits the obvious-affection wearer. The right pick matches the recipient's identity, not the loudest visual.
Wearability past the birthday. A snail birthday gift t-shirt that gets worn once and then lives in a drawer is a worse gift than one in the regular weekly rotation. Designs grounded in everyday color palettes (faded sage, dusty plum, washed-out indigo) tend to outlast novelty-bright prints.
Order-by-date lead time. Amazon Merch on Demand designs are print-on-demand, so a snail birthday gift order benefits from a few days of buffer between click and party. Earlier is safer than cutting it close to the candle.