I'm not sure if this is just me not knowing or a issue with my install. I can't seem to get the classifieds to work using the paid option. When I sign in with a test account and go to place an ad. I fill out the ad, put the duration down and submit. The ad is submitted without the need to pay for the ad. It's also weird that I can't see the test user's ad from my admin dashboard but when I click on the classifieds link in the menu bar I can see all ads on the site.
I've set up the sandbox and can purchase credits through the test account if I manually go to the checkout page.
You can view the issue here -