If you use the standard export dialog it is possible to specify those exact dimensions, but I think you are being overly concerned. In reality, you don't have to resize at all. You can send the full-sized JPEG images and order 4 x 6 prints, and you will get what you are expecting. If you want to conserve space and save upload time you can resize them to 1200 x 1800 pixels. Their printers will compensate and size the images to provide you with their 4 x 6 prints. The little bit of difference Costco is mentioning will have absolutely no impact on the quality of your images. Don't worry about it.
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