I think there might be a confusion in terminology.
You want a “sign up” page to have Paypal buttons to purchase. This would be your Membership Option page (if you want). The page where the users are directed to CAN definitely have those buttons to purchase your membership or product.
The Login page is just the page where a registered user would be sent to LOG IN. If a visitor was to land on that page, they would be prompted to LOG IN, or, at the bottom of that page, they could REGISTER (that would be a free registration for Level 0 by default)
So the path for a visitor would be this:
– comes to the site
– tries to access a restricted page/content
– is sent to the Membership Option page where you can give the pitch of “please, buy this, or register”
On that page, you can have 3 things:
– either Paypal buttons so the visitor can directly purchase from that page
– either a link to a sale page where you can detail more about your products and have a Paypal button there
– or a regular link sending the visitor to register for free (sending to the http://yoursite.com/wp-login.php?action=register page)
Have a look at my site for an illustration: http://scrapbookcampus.com/
If you try to access this page: http://scrapbookcampus.com/element-creation-index/plaid-paper-1/
you will be sent to the Membership Option page because it is protected.
From that page, I have links to sales page, but I could also have placed Paypal buttons directly on that page (I just wanted to be pushier).
Do you think this would clarify what is happening with those pages you are talking about?