@LauraBow thanks for trying.
Any clicking on "buy sim points" or the world with RL currency on the site, takes you to an EA page advertising its other games and a pop up that says to get the EA App because Origin is closed. Clicking on buy simpoints in the Origin launcher opens an old version of Internet Explorer that wants to change to Microsoft Edge and may or may not open the Sims 3 webpage, but then does the same thing as I described before.
The only way to buy simpoints and rl currency things is through the EA App.
So I had to download the EA App onto my laptop, install the base game on it, then buy simpoints, and Barnacle Bay through that. Then go on my PC and download it from the site into my Origin launcher on there. I probably could have downloaded it into the EA App on the laptop and save it onto a USB and then put it on my PC but that would be an extra step.
The switcheroony between the laptop EA App and the PC Origin launchers is because the EA App causes Store content, including worlds, to keep on uninstalling itself in between restarts. The only way to get it reinstalled after that is by redownloading. The EA App seems to install a mac version of Sims 3 even though it is meant to be a Windows one. Sims 3 is unplayable more than once on PC with the EA App. Luckily I do have a laptop to do that on eh.