One Light all-in-one unit, batteries included with 2 units

I recommend putting a pull tab under the battery before mailing a light-up card. Be sure to keep the release paper from any foam tape that crosses the tab in place, so the tab doesn't accidentally get locked in. I stamp or write "pull" on the tab. Here is a short video that explains the process... https://youtu.be/Fgg4gftO5lI?si=QO2h3igf6dUpPN-k

When mailing light-up cards, as long as you have the pull tab in place to keep the battery from draining, you don't have to do anything else special, unless your card exceeds the one ounce weight limit or 1/4" thickness restriction. Our lights are about 1/8" thick (well under the letter limit). If a card is thicker than 1/4" or gets mailed in a rigid mailer, it doesn't count as a letter any longer, and the post office will charge package rates. 

Our lights are very lightweight. A 5 pack, including batteries and packaging weighs less than one ounce. If you make your cards with a lot of layers of cardstock, you can get up to an ounce pretty quickly though (cardstock gets heavy). It's a good idea to weigh finished cards before mailing. If they are over an ounce, add an extra ounce stamp. 

The battery chemistry is lithium metal, not lithium ion (much less potential). As such, you don't have to put UL stickers on shipments with only one battery inside.