Ultra-light Online Purchase
Tumblr uses a small dynamic overlay for purchasing blog themes:
This lightweight approach seems ideal for purchasing single items that don't need to be shipped.
Add to Cart Bad Browser Back Buttons Card Stack Clear Button Contextual Menu Dates Downloading Drop-down Menu Dynamic Dynamic Menus Ecommerce Edit Email Fade Fields Filed Help Filmstrip Navigation Filtering Fluid Display Forms Global Navigation Help History Home Page Inactive Values Layout Legal Agreements Linear Navigation Login Maps Menus Multiple-select Navigation Paging Palette Password Popup Portal Product Availability Product Details Product List Profile Progress Bar Quick Details Radio Buttons Rank Ordering Registration Scrolling Scroll Wheel Search Selection Sliders Tabbed Browsing Tables Text Editing Web Application Wiki Wizard