Epson Stylus Color 640 User Manual page 65

Set the thickness lever to the + position when you print on
envelopes or if ink smears when you print on other thick media.
Open the printer cover and position the lever as shown below.
Then close the cover and print. (Make sure you return the lever
to the 0 position before you print on regular media.)
Use ink jet papers within one year of purchase. Use glossy
media and transparencies within six months. Return unused
sheets and envelopes to their original packages as soon as
possible. See Paper for more media storage information.
Printing on Special Media Page 65