A standard-sized letter envelope costs from 49 cents to mail domestically and from $1.20 to mail internationally, as of September 2015. Postcards cost 35 cents to mail within the United States. Generally, postage rates vary, depending on factors such as content, size, shape, weight and distance the mail travels.
Other factors that affect the postage rate are quantity, the sender's non-profit status, delivery speed and the mail entry point. Bulk mailers also need a permit and must pay an annual mailing fee per class of mail.
In general, the USPS determines postage rates by considering the balance between the value a sender obtains from using its service and the amount of work the sender does to prepare the mail or parcel. Therefore, the more work a sender does, the lower the postage rate. This way, the USPS saves on labor costs that it passes onto its customers in the form of good commercial rates.