What is the maximum size for flat mail to be sent via USPS?

The maximum size for flat mail sent via USPS is indeed 12 x 15 inches. This dimension is critical for ensuring that the items can be processed and delivered efficiently within the postal system. Flat mail, which includes items like large envelopes and postcards, needs to fit within specific size guidelines to qualify for flat-rate pricing and processing.

When items exceed these dimensions, they may fall into a different category for mailing, such as parcels, which are subject to different rates and processing protocols. Understanding this limit helps mailers avoid unexpected costs and ensures their mail is processed efficiently. Therefore, recognizing the size limits is essential in postal services for both efficiency and cost-effectiveness.