Can't login to purchase prime license even though I have an account. Have reset password several times and still can't log in.

I want to buy Mathcad Prime but I can't login to purchase. I have created an account. When I click "buy now" it opens a new tab with a login screen. I enter my information and it says "invalid username or password". I have reset it several times and after completing the reset I can see that I'm logged in because my name appears in the upper right hand corner. When I go back to the store to purchase the license, the same login screen opens up, I enter my email and newly changed password and I get the same error. What am I doing wrong here?