You are right - they seem to make it really difficult to do this - I was having the same problem and, after about a hour! of googling found the following information hidden deep in the napster site which is only accessible AFTER you have clicked on one of the 'get napster unlimited buttons'!. I haven't tried it yet - just about to - hope it works for me - and you :) It looks like you have to go through the process of creating a subscription account part way when there will be some kind of option to change it and not have to enter any card details etc. Misleading much!
INFO TAKEN FROM NAPSTER SITE AT
http://napsteruk.cust...napster%20mp3%20store
"It is not necessary to download the Napster software, become a subscriber, or even take a free trial of Napster if you would just like to purchase MP3 tracks and albums from our catalogue of over 10 million songs.
To become a Napster MP3 Store member, just click on the Get Napster Now button on the Napster homepage. On the registration page, choose a Napster Unlimited membership and click Continue. Enter your email address, create a password, then click Next Step. On the payment details page, just click the "Just want to buy tracks and albums in MP3?" link then "Continue to the Napster MP3 Store."