The art customers send us may include text. That text frequently is uploaded to us without creating a version that is print ready. When our system opens your file it may try to replicate the fonts you used if we do not have the same exact font and this may alter your art. To get around this you should convert text to outlines/graphics/vector art (all 3 are the same thing). Our example of how to do this is in Adobe Illustrator.
How do I outline text in Adobe Illustrator?