The checkout page of your Social Commerce theme, when viewed on Facebook, is not long enough and cuts off the payment options
Social Commerce 1.5 Now Has Seamless Integration to Facebook Without Requiring a 3rd Party Tab App. Please update to and Re-add your tab to take advantage of new features.
Setting Height in Thunderpenny
Edit your tab and click the Remove Scroll Bars checkbox, which will display two sliders. Adjust the height scrollbar while previewing your page until it is long enough. You may need some trial and error to get it just right.