The United States Postal Service has pre-determined envelope sizes from which they base their mailing fees. If the envelope that you will be sending exceeds the maximum size of a specific envelope type, the price will be based on the next bigger size, of another envelope type.
So for instance, if you are sending a letter in an envelope that has exceeded the maximum allowable letter envelope size; your envelope will be regarded as a Large envelope which will then be priced a bit higher.
There might also be instances where you will be asked to place your mail in another envelope because what you have is not within the allowable envelope sizes – USPS.
For your reference, the following are the allowable envelope sizes:
Letter Envelope – Minimum: 3.5 inches in height by 5 inches in length; Maximum: 6 and one-eighths inches in height by 11.5 inches in length; Maximum Thickness: one-fourth inch
Large Envelope – Minimum: 6 and one-eighths inches in height by 11.5 inches in length; Maximum: 12 inches in height by 15 inches in length; Maximum Thickness: three-fourths of an inch
USPS Flat Rate Envelopes
Flat rate envelopes are special envelopes that you can only get from USPS. These come in pre-determined sizes and are usually printed with: Express Mail or Priority Mail.
Sizes are as follow:
Express Mail – Regular: 12.5 inches in height by 9.5 inches in length; Legal: 15 inches in height by 9.5 inches in length
Priority Mail – Regular: 12.5 inches in height by 9.5 inches in length; Legal: 15 inches in height by 9.5 inches in length; Padded: 12.5 inches in height by 9.5 inches in length
Gift Card Envelope – 10 inches in height by 7 inches in length
Small Envelope – 10 inches in height by 6 inches in length
Window Envelope – 10 inches in height by 5 inches in length
Apart from special USPS envelopes, there are also boxes that you can get straight from USPS. Sizes for these are the following:
Small Box – 8 and five-eighths inches in length by 5 and three-eighths inches in width by 1 and five-eighths inches in height
Medium Box – 13 and five-eighths inches by 11 and seven-eighths inches by 3 and three-eighths inches; or 11 inches by 8.5 inches by 5.5 inches
Large Box - 12 inches by 12 inches by 5.5 inches
Large Video Box – 9.25 inches by 6.25 inches by 2 inches
DVD Box – 7 and nine-sixteenths inches by 5 and seven-sixteenths inches by 1 and three-eighths inches
Note: Flat rate for the Large Video Box and DVD Box is applicable for international shipping only.