Usps First Class Letter Size Restrictions. All postcards and envelopes (or flats) must be rectangular, otherwise an additional charge may apply. I have sent thousands of these and never had an issue.
Contents must weigh less than 70 lbs. Requesting a certified mail receipt or other proof of delivery will add to the price. To give you an idea, it may cost around $0.98 for the first ounce of a large envelope and $2.61 for the first 3 ounces of a small package.
Regardless of the size of the mailpiece, the maximum mailable weight of any mailpiece is 70 pounds.
Well, this is one finest mail service that offers fast delivery at an affordable price. “if it fits, it ships.” considering the nature of flat rate shipping, this means there are relatively few usps shipping restrictions on flat rate. How many stamps you need to mail your letter depends on where it's going, its size, and its weight. To give you an idea, it may cost around $0.98 for the first ounce of a large envelope and $2.61 for the first 3 ounces of a small package.