What characteristic affects the pricing of first class mail letters?

The pricing of first class mail letters is influenced by multiple factors, including size, shape, and weight. Each of these characteristics plays a vital role in determining postage costs.

When considering size, letters that exceed standard dimensions may require additional postage due to the need for special handling. Similarly, the shape of the mail is significant; non-standard shapes can also incur higher costs because they don't fit into automated processing systems efficiently. Lastly, weight is a direct factor in postage pricing, as heavier mail requires more resources for transport and delivery.

Since each of these characteristics can affect postage costs, the correct answer is that all of them together contribute to the pricing of first-class mail letters. This reflects the comprehensive approach taken by postal services to establish fair and effective pricing models based on the physical attributes of the mail.