Printing a mirror image for a tshirt

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Your printer might be able to do this automatically. Otherwise, select your image (floating/fixed on page) & use the metrics inspector to flip it. This picture shows a text box flipped, but the same applies to any object.

but the flip box is greyed out. Why would this be?

Most likely because your image is in-line (moves with text) instead of fixed/floating on page. Select the object, go to the Wrap Inspector, and fix it.