A special t-shirt?
Runners usually don't wear the race t-shirt on race day. If they like the shirt, they'll wear it on training runs or at a future race.
When Nike produced its Run Hit Wonder 5k & 10k, however, every runner wore the race shirt because bib numbers were printed on them, making each a one-of-a-kind piece of apparel. There was no other option (and it made a great photo op).
The race was a promotional venue for Nike, so the cost of the individualized shirts was absorbed by the event.
A number of races are using that same concept, but not paying for it themselves. At the Rock N Roll Marathon in San Diego CA, Fox Cities Marathon, in Appleton WI, OBX Marathon, in the Outer Banks of NC, and several other events, runners have the option of paying for a microfiber shirt with their bib number printed on it.
Shirt costs vary from about $26 for singlets to $35 for long-sleeve and runners pay the shirt company directly - in addition to paying an entry fee to the race. The race promotes the t-shirt company in its marketing materials.
Those who don't want this shirt still receive a race shirt and bib number from the race. The races use chip timing, so a bib number with a tear-off is unnecessary.
Let's see how popular this offer is among runners.