I want to go for the achievements for donating pumpkins to Fantine, but I can't figure out how to donate my pumpkins. Halp D:

You keep the pumpkins in your inventory, and go here: Fireside Fantine and each individual pumpkin will show up for you to click on and donate. If you have several of the same pumpkin you can click on one and on the page that appears, keep hitting refresh until you have donated each of them. (It will tell you "You can't donate that item" or something like that when you run out of a pumpkin item)

You can donate both uncarved and carved pumpkins. So that means all pumpkins that were floating items on the site during Morostide or have been in the past, and the pumpkins you can carve from them (so no other pumpkin items). If I'm wrong about this please someone else correct me, though I am fairly sure it is the case.

You can also reach Fantine by going to the News page and clicking "Fireside Fantine" in the right sidebar under "Check out". I believe in the past you could access this screen through Fantine's regular shop, but that does not seem to be the case now. I may be wrong about that though.