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I think that you have the trays still on their default settings. The settings for Tray 1 or Cassette 1 need to be changed to 8 x 10 ", if they are printing on a different size.
There are two main places to look for such a setting. The first is on your computer (given that you have installed the Epson 3640 drivers correctly), and should be various places depending upon whether the computer is a Windows PC or an Apple computer.
(On a Windows PC look in
Control Panel - Devices and Printers
and right-click on your printer's logo there, and select Properties. Look for 'Paper Size' there along with a Paper Tray specific option, or similar setting. Change it to 8 x 10, then save the changed setting by clicking on OK until the Properties sheet(s) close(s).)
The other setting would be the Paper Source setting.
The other possible setting would be on the printer itself. But I doubt there is one.