Hello Daniel,
Gotcha -- when you are logged into BidSlammer, at the top of the page, there should be an "Upgrade" and rocket ship icon - if you click on "Upgrade" here, you should be brought to a similar page as the /pay URL.
Please let me know if you run into any issues with the "Upgrade" link at the top of your logged in user page?
BidSlammer Support
bidslammer