In s2Member > PayPal Pro Forms > Wrapping Multiple Shortcodes, you’ll find this solution:
It is also possible to link to a Pro Form and pre-select a specific Checkout Option that appears in the list. Starting from the first Checkout Option in the list (we call this Checkout Option 1) you can choose which Checkout Option number you want to have selected by default. This is accomplished by linking to any Post/Page on your site which contains a Pro Form Shortcode; and then adding the ?s2p-option= variable onto the end of that URL (as seen below).
This example would pre-select option 2.
The absolute default Checkout Option is always the first one (Checkout Option 1).
This would pre-select option 1 (but this is NOT necessary, because it’s the default already).