For optimum print quality high quality JPEGs are fine provided the image is near or at the required print size.
When exporting your final image for print, we recommend using a Photoshop compression level of 10 (11 & 12 offer no quality benefit but will increase file size) and a quality setting of between 93 and 100 when exporting from Lightroom.
Edits and adjustments to files carried out in LR or Photoshop should be done using RAW, Tif or PSD formats. It is best practice to only export to JPEG once editing is complete and the file is ready to submit for printing.
If you would like us to resize the file for larger prints then we recommend you send the file in Tiff format, please note this may mean a much larger file for upload.