Basically all the information is below...
We are going to use the SMS upgrade system.
How it works is this...
A person buys the main product on our subaccount 2.
Upon successful purchase they are redirected to a page similar to this... (wordpress page)
[url removed, login to view] to be shown a special offer to upgrade.
Which the product is actually contained on subaccount 3 (upgrade)
If they click the YES button they are charged for the upgraded product and redirected then to the registration page.
The basic button code is...
No Thanks I Would Rather Go It Alone And Pass On This Special Offer
As you can see the subscription ID will have to be passed to this page and the redirect upon successful purchase will be set.
All the documentation on the SMS is linked below.