What happens if a letter size envelope is thicker than 1/4" but is uniform in thickness?

When a letter size envelope exceeds the thickness limit of 1/4 inch, but maintains a uniform thickness throughout, it is classified as a flat (large envelope). The USPS has specific size and thickness requirements for different mail categories, and an envelope that is thicker than 1/4 inch falls outside the parameters established for standard letters.

By defining it as a flat, the USPS adjusts the postage rates and handling processes that apply to such mail. Flats are generally subject to different rates than letters, and understanding these classifications is vital for ensuring correct postage and appropriate mail handling by the postal service.

In contrast, the other options would not apply because the envelope does not qualify as a parcel, which typically encompasses items with different shapes and weights or that do not meet the flat or letter criteria. Furthermore, since the envelope is still uniform in thickness and does not exceed the maximum dimensions that would render it non-mailable, it can indeed be sent at a prescribed rate. However, because it surpasses the specific thickness for letters, it correctly falls under the classification of flats.