The "No. 10" envelope (9.5" x 4.125") is the most common choice.
You can get envelopes from the USPS either for free (for specific premium services) or by purchasing them for standard mail. 📦 Get Free Envelopes usps buy envelopes
The USPS provides shipping supplies if you use their expedited services. You can order these on the official USPS Store. The "No
Specialty sites like Envelopes.com offer jumbo or custom sizes. 📏 Size & Price Rules usps buy envelopes