Does it take additional postage for a 6 X 9 inch envelope?

Size limits:

- Rectangular; length is the dimension parallel to the address.

- At least 3-1/2 inches high by 5 inches long by 0.007 inch thick

- No more than 6-1/8 inches high by 11-1/2 inches long by 1/4 inch thick

- Up to 3.5 ounces

- Surcharge will apply for nonmachinable letter

So, as long as it is rectangular, NO. The maximum without additional postage is in bold. If it is larger than that, yes additional fees apply.