Interesting business model...Promoters design shirts then advertise to their customers to buy them? Amazon gets the $$ and the promoters get brand awareness and a few cents of royalty...
Their example shows that a shirt selling for $19.99, the designer/promoter would get $8.89 royalty. More than a few cents and not a bad deal for not having to deal with the actual production.
And if you use anything but their lowest quality shirt and make it double sided then your cut is dropped to about $3.40..true it's more than a few cents but still a pretty small cut per shirt.