I have NEVER seen such a convoluted site such as Adobe. This is pathetic. I have spent hrs today trying to find how to buy a subcription. After being sent around in circles, I finally found a place and was able to purchase. I tried to download, it wouldn't work...just a blank screen. Chat agent finally gave me a link to download. It was a trial version, but he said it would convert when I finished loading. It asked me for a serial number or "try". I couldn't find serial number. Next chat agent sent me on a goose chase to find. Not there. Finally says I need to wait 12-24 hrs. Finally found my way into this forum. AFTER many many tries. Who designed this site? HORRIBLE.
I don't know who created the site or what tools they used, but you aren't the first person to complain. That being said, I have used several programs that began as trial versions, and they all ask, as the first question, whether I want to try it out for free (hit the Try button) or buy (enter a serial number). It's the same program either way, but the serial number unlocks the program from the trial mode, which will time out after a certain period (like 30 days, for example).
I bought CS3 as a download, but that was a few years ago, so I don't remember exactly what the procedure was. Most programs will take you to a purchase page, where you enter your personal and billing information, and they send you a serial number via email. When you launch the program, you just copy the SN from the email and paste it into the field. I imagine you can take as much time to complete the billing part as you are allowed by the trial period, so if it works in trial mode, it's not a deal-killer problem like you would have if the program wasn't fully functional. And I'm just guessing here, but maybe the 12-24 hour bit is the time between you submitting your info and it being verified, which triggers the sending of the SN to your email. Still, I feel for you, but that's about all I can do.