Which service allows for online purchase of postage?

The option that allows for the online purchase of postage is Click-N-Ship. This service is specifically designed for customers to buy postage labels for mailing packages directly from their computers. By using Click-N-Ship, users can print postage at home or in the office, which enhances convenience and saves time compared to visiting a post office.

Other options, such as Priority Mail and Express Delivery, refer to mailing services rather than platforms for purchasing postage. These services are types of shipping options provided by the Postal Service that involve delivery speed and package handling but do not facilitate the direct purchase of postage online. Package Tracker, on the other hand, is a tool for tracking the status and location of mailed items, and does not involve purchasing postage at all. Therefore, Click-N-Ship is the clear choice for those looking to buy postage online.