I'd check the file size of the picture that you've uploaded. At a guess I'd say you've tried to resize to specific dimensions and you've gone wrong with the sampling and you have a really small file size.
I've had thousands of prints done online over recent years and never once re-sized an image to accommodate a specific print size. I've reduced file sizes to speed up the upload process for 6x4s - it matters when there's 500+ to do. In those cases I've reduced the file sizes to around 2 to 4mb each. For bigger prints - 16x12 - I've uploaded files in the region of 5 to 8mb, some 9 or 10. For some 30x20s the file size was 10mb - the biggest the printer [Photobox] would take for upload. All have come out good.
Once uploaded, see how they fit on the web page template and decide if you need to select crop to fill paper, or shrink to fit. You'll want crop to fill for your canvas I expect. Shrink to fit will likely give you some white borders. With crop to fit you may lose a bit of a couple of edges, sometimes hardly any at all. Decide if it's a problem if a bit of an edge gets cropped.
So for 18x12, to be on the safe [as I don't want to get in to trouble here!] tend towards a 10mb file from your picture management prog and it will be ok. And it being a single file, it won't matter that it takes a couple of minutes or so to upload.
