Help on 1883 Jackson 4c green

If it is soft porous paper, it's probably not worth the expense to get a certificate, since the stamp was normally issued on soft porous paper.

While it looks like soft porous paper from the pic, your description of onion skin paper suggests it might very well be something else.

Dip face down in watermark fluid and see if you can see a bunch of very small fibers. If so, it is likely India paper (i.e., stamp cut/perforated from a plate proof). The fibers would be Bamboo fibers. The impression seems quite sharp, however I do not have a plate proof to compare the sharpness.

India paper will tend to wrinkle when wet. But I would suggest doing the watermark fluid dip as a primary method (alternatively, you can use high magnification if you know what the embedded Bamboo fibers are supposed to look like).