1 person found this helpful
I have never heard about the alloy photo plugin before so I googled it. I print to costco all the time but always export myself and upload the resulting jpegs. I would cut out the plugin first. There are two things that you need to do first which are the fact that you should make sure to crop to the right image size you want to print at either in the export panel or by printing to jpegs from the print module (the simplest way if you don't want to create a lot of extra virtual copies). Then you want to make sure that you have enough pixels left over. If you cropped your image heavily, you might not have the requisite 1800x1200 pixels for a 6x4 photo at 300 ppi. That's probably what the plugin complains about so check that first. Just hit the I key to cycle between different info displays. One will show the dimension of the original image in pixels.
A good workflow for printing a series of images at costco at 4x6 is to set up a 4x6 custom dimension print layout in the print panel with one image covering the entire area (zero margin). Set the image to scale to fit and autorotate. Choose sRGB as color space and set the resolution at 300 ppi. Choose medium or high sharpening for print at either glossy or matte paper depending on what you will print on. Low sharpening is not aggressive enough for costco's machines. Then print to jpegs and set your internet browser to costcophotocenter.com. Upload the images (make sure to choose the full resolution option) and print the entire album at the size you set in the print panel. Do not print these at any other size. This will give you high quality prints and will not require a plugin. You also will never get complaints about the image resolution as Lightroom will simply upscale smaller images.
If you know what you are doing and calibrate your computer's display, you can get better quality still by using the icc profiles costco makes available at drycreekphoto.com in the print panel. Make sure to select perceptual as the rendering intent (the relative intent can cause posterization) and export. Then you want to strip off the icc profile as it adds over a megabyte to every jpeg and the printers ignore it anyway. I do this using a simple photoshop droplet but there are many other ways. You also want to turn off automatic color corrections in costco's photo center. Doing this gives very good color reproduction.