The Lightroom Screen and Print sharpening are just that...you won't be getting optimal sharpening for print if you export with Screen. You need to find out for a fact whether or not any output sharpening is being applied by your service provider. If it is, don't apply any output sharpening on export. If it isn't apply the correct output sharpening for printing: Print (and test to see if Standard is optimal for you, it is for me).