MOD POST: Reminder: Shipping Safety

When putting together packages, please be aware that some items may require special packaging or declaration; other items may be prohibited.  This is an international community, and restrictions can vary by country, and by shipping service.  Generally speaking, anything potentially Explosive or Flammable may be illegal to ship (this includes nail polish, batteries, aerosols, certain types of paints, and perfumes with an alcohol base).  Please use good sense when putting together packages and labelling them.  If you are unsure what is legal to ship within your country, or from your country to another country, please check with your postal service.

As examples, here is the UK's Royal Mail and the United States Post Office lists of prohibited items.  If you have a link to a similar list for another country's postal service, you are invited to share it in the comments.

ETA1:  Don't be discouraged from filling wishes for these kinds of items!  You can send a gift certificate to an online or local store that sells the items in question (do verify what brick-an-mortar stores the person shops at).  You can also buy the items online and have it shipped to their address as a gift- retailers who sell such items have already dealt with the government's red tape so that they can legally ship their merchandise.

ETA2: from the comments: