What are some rules about changing photo size in photoshop (dealing with dpi) please read details?

I have an image that is 10 by 6.6 inches at 72 dpi (1016K). I change the canvas size to 4x6 and scale the image some (one of my methods of cropping. I then upload it to the website to order prints and it tells me it's not high enough quality. However, if I upload the original, it can be printed at 4x6. When I scale it using the free transform tool, does it reduce the pixels? How can I crop it to 4x6 without messing up the quality? I don't want to use the crop tool because I like to make the canvas size smaller then move the image around to find the best composition.What are some rules about changing photo size in photoshop (dealing with dpi) please read details?
There may be compression options pre-set in your Photoshop. If you can't find them, or there aren't any set, try using a ';lossless'; image resizer.

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