Hi Stedracu,
My preference has always been to create different fee levels for members. Economy, Premium as an example. If you go into admin panel/Settings/Membership Levels, you can then add new membership levels, including prices and privileges.
Furthermore, when you go into admin panel/settings/payment settings, remember to add your PayPal Account email address in Business section of admin panel/settings/payment settings. Otherwise, the process won't work.
When you save your settings, remember to also click Validate Saved Configurations otherwise it won't work either. Finally, I would also recommend you uncheck the other payment options and save those settings as well.
I hope this helps. As you can tell by the way I've written this, I am not a software programmer.