Either you have an odd box or I have.
I was using 13" x 19" earlier today and it really curled right after printing, but seems to have settled down after drying on a flat surface for couple of hours.
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I haven't tried these papers yet, but will be receiving them on Monday. But I have a suggestion for you.
Epson recommends that all inkjet prints on photo paper be placed between plain sheets of paper. The paper soaks up the gases -- these papers "out gas" for at least 24 hours. My experience is even longer. I don't think HP has ever made that recommendation, and they should. Epson says that if you frame a photo ink print immediately, the out gassing will fog the inside of the glass.
Here's what I do, and it might solve your problem. I buy large pads of drawing or tracing paper at art supply stores. I just put the prints inside the pads and use the various pages as interleaving. I use plain drawing pads most of the time, as the paper is thicker and can absorb more gas. If the surface is delicate, like Harman, I would use tracing pads. The weight of the pad really helps to uncurl paper. Something that curls badly could be put in the bottom of the pad, with the weight of the pad on top of those prints, and you can always put one pad on top of another.
If you leave prints in such a pad for a couple of days, the drawing paper in the pad gets quite wavy and curls from the outgasing.
That is very evident with thinner tracing paper.
I tend to leave prints inside such pads for three or four days, even though I believe Epson says one day is all you need.