My first assumption is [shirts -stripes]

Of the first ten hits: Amazon: 7/10 aren't shirts (but all are stripeless) Google: All shirts, one is checked Bing: All shirts, all contain stripes in description.

Not exactly glowing for Amazon, clearly unsupported by Bing.

Like Google, Amazon is supposed to support the - operator, but, also like Google, it's not clear whether it does anything at the moment. As others here have suggested, maybe "solid shirts"?