Add a way for site owners to add a direct link to the checkout page, that is smart about what level the Members has already.

[kimwhite]

Is your feature request related to a problem? Please describe.
Right now, If I want to directly link to an Add On Package Checkout page, I could add:

http://addonpackage.local/membership-account/membership-checkout/?ap=18
or
http://addonpackage.local/membership-account/membership-checkout/?level=2&ap=18

However, the 1st link would find the default level (usually 1) even for an existing member.
The second would force level 2 no matter who clicked it. And, change their current level.

Describe the solution you'd like
The logic on the Checkout button on the Course page does this. Can it be used in a shortcode or link form on other pages?

Describe alternatives you've considered
Right now, you can use the visibility setting to create multiple "buttons" with links that will only appear if it's the right conditions.