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Resolves #62

How to test the changes in this Pull Request:

1. Create a checkout level with several ( at least 3 or 4 ) different payment plans.
2. Add to the checkout level link pmpropp_chosen_plan=2 as a param
3. Observe that the third plan is selected. Even if there's a default plan.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you successfully run tests with your changes locally?

Changelog entry

Enter a summary of all changes on this Pull Request. This will appear in the changelog if accepted.