Add the subscriptions as you want in your Avangate dashboard. When you're done, generate links for each subscription and enter those at Admin > User Plans > Edit plan > Order URL. A subscription always has a minumum trial period of 7 days and looks like this:
Then, generate upgrade links in Avangate for each plan to the next and enter those at the Upgrade URL field. An upgrade link looks like this:
Looking at the urls, you can see that the product id is
4534429. Enter this numeric id at the Product ID field.