The My Coins balance that appears on the sticker/theme/emoji purchase screen includes your purchased Coins and any Coins that you can get by exchanging your Points.
Points are exchanged for Coins at a rate of one Coin for every two Points.
Note: The exchange rate may differ depending on the country you're using LINE in. See this Help article for details.
For the steps to purchase stickers, themes, and emoji with Points, see this Help article.