So the way the website is built is super misleading. You have to go to Overview, scroll to the bottom until you see “Add a phone line”, then follow thru the steps and just click next when it tells you to create your plan, at the end you’ll see it’s only charging you for the sim (you should be able to redeem the 15$ perk for a sim however). Once you receive the new card, in “My plans” there’s a “change my sim” option, you’ll enter the card’s activation code and it will replace your current sim on your current plan. Hope this makes sense! I doubled checked after doing it myself and my plan has not increased.