Mark Sirota , Aug 05, 2010; 03:03 p.m.
Rather than adding borders, I would suggest cropping your shots to match the aspect ratio of the paper you plan to print to. If you plan to print 4x6 or 8x12, that's 1:1.5 as they said. If you plan to print 5x7 or 8x10, those are different... The crop tool has presets for these aspect ratios, or you can set your own.
If you really want to pad out your shots with borders, there are a few ways, none of them elegant. The best two for a Lightroom workflow are to do it in the Print module, and print to a JPEG file which you'd send to Costco, or use the LR2/Mogrify plug-in from Tim Armes on export.
Is their profile for soft-proofing, or do they want it embedded in the file? Some labs want you to soft-proof using their profile, then send them a file in sRGB or Adobe RGB. Some want you to embed their profile. You need to ask what they want.