Help with an unknown semi-transparent layer!

Hi,

I'm working on an image to print onto t-shirts for our holiday.

When I upload it into vista print it says there has been a semi-transparent layer detected which may affect the quality of printing (as they are being printed on black t-shirts - they put a layer of white behind to ensure best print quality)

The problem is I can identify what the semi-transparent part is! I've checked all layers and they are set at 100% opacity.

Here is the file if anyone can help? I don't want to order and pay for 15 of these t-shirts to find they have printed incorrectly!

Thanks in advance!

http://www.2shared.com/file/6483268/aa6957fe/TShirt_Zante2009.html

Why not just flatten the image for printing?

Why not just flatten the image for printing?

I think so..

Or, if you absolutely need the transparent background, you could merge all the layers.