Cambio isn't charging the vendors a fee, and they aren't keeping tabs how much is sold, so the 15% donation runs on the honor system...
Should be okay though. Their Halloween market raised 12,000rmb for the Chi Heng, and Cambio's founder Sebastian Martin says that this year they are expecting double the turnout.
In addition to the vendors' donations, the market has options for attendees as well. You might have guessed it: they have a raffle with prizes -- 100% of the proceeds goes to XinXing -- everyone's got a raffle these days. Gotta have that raffle. Good thing is, they've got some non-cash ways to give too. You can donate used (but not unsoiled/undamaged) clothes to Green Initiatives' Fibre Project, books to the Library Project or swap books with Lohaus, give away old electronics to Netspring, and sign up to volunteer with Hands On.
If you're stuck in Shanghai over the holidays and didn't get enough Christmas spirit this year, then Cambio's Neverland Market looks like a good one to check out. It runs from 11am-7pm. No cover. Happens at 1107 Yuyuan Lu, near Jiangsu Lu.
To see the other Christmas markets going on in town, peep our Christmas page.