the concealers themselves are  the size of a felt tip pen about 10mm in width,  but the boxes they come in take the dimensions to about 25mm.   ... Any ideas?

The obvious solution is to supply the concealer AND the box, but pack them separately, by removing the concealer and flattening the box.

That's the way that I do things to get under the 20mm limit.

BTW - a trick to sending something like a pen. Get some polystyrene (white) foam the same thickness as the item (eg: 10mm). Cut a rectangle of the foam, then cut a slot in the rectangle that is large enough for the item.

Now sandwich the foam between 2 sheets of cardboard, and tape it all up into a little "packet".

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| foam |  ITEM IN HERE  | foam |

=======================

Now flatten the original box, and put it in a large plastic bag, folded over. Tape the bag closed then tape the bag alongside the little foam packet you just made.