As long as the postcard stays within the green "Safe Zone" there should not be any issues with something getting trimmed off.
The safe zone dimensions for 4x6 postcards at 300 DPI would be 1162.5 pixels high (3.875*300) and 1762.5 pixels wide (5.875*300).
The safe zone for an 6x11 postcard at 300 DPI would be 1762.5 pixels high (5.875*300) and 3262.5 pixels wide (10.875*300).
โ