Hi,
I started with a 3000 x 3000 @300dpi image that has lettering on it. When I look at the image on the product it looks like the letters are jagged but when I zoom in on the uploaded image, they are completely smooth. The image was uploaded in png format. If the uploaded image looks smooth and it is uploaded at such a high resolution, what more do I need to do to make the letters look smooth on the shirt? PLEASE HELP!

This is the product:
http://www.cafepress.com/truth4life.407958650

Without seeing what you are seeing we can only make wild guesses. The wild guess would be that it will be fine. The problem may be in just how the automatic thumbnail was created.

Its just how CP resizes the images. Some of mine don't look to great on the thumbnails but the enlarged version and what actually prints looks fine. Also I've noticed that if you have a high concentration of red in your design, it looks blurry on the thumbnails....

thank you everyone for your advice. I'll just assume it will print fine.

When all else fails, you can try buying the design on something like a value t-shirt to see if it looks okay, without spending a huge amount.

I've seen people say that they did a montage or grid of several designs on a single item (the design would obviously be smaller that way) to test things like color accuracy.