I powered down my phone then turned it back on. Then I turned Airplane mode off and back on. That fixed the issue for me this morning. Hope that works for you too cuz it was super frustrating.
I tried signing up for the $40 75GB but when I selected to have it start when my current subscription period expires, the site crashed. I logged in again and now it's is only offering the US Canada promo for $50. Is that a bug?