jc524 try doing it from an incognito/private browser instance. That's what worked for me. Hope it works for you as well.
I ended up trying to buy it in-game rather than the web store and it gave me a prompt saying something like "you have a pending purchase, would you like to cancel the pending purchase first?" I said yes & ended up buying the pack in-game. I got the secondary $75 pack in the webstore using the incognito browser you mentioned & it worked
I'm having the same issue - but im not getting the cancel pending purchase option....
