In AMember 3 you could set a link using "signup.php?price_group=1" to direct a user to a single product / product group. I don't see anywhere how you do this in aMember 4? Also I enabled the plug-in for "Cart", but I don't see any instructions for how to use/access this? Thanks!
I will respond to this for others who may have the same question (based on email support response): Please visit AmemberCP->Configuration->FormsEditro, then create news signup form, edit it and configure Products brick so it shows only one product. After that you will get link to signup form with single product.
About the "Card" plugin - once module is enabled, you can access your shopping cart at http://example.com/amember/cart/