Upload & Sell: On
Select your image in LR and go to File->Export
In export options, under image resizing check off the 'resize to fit' box. Select 1920 as the size on the long edge.
Under the 'File Settings' tab, make sure JPG is selected and check off the "Limit file size to" box, enter 1800 as the value.
A JPG of the appropriate size should appear whether you specified the output folder to be.