Lines were too thin

The quality of the product was great but I guess the lines on my art and one of my fonts were too thin (1-2 px). Portions of the artwork (in white) are barely visible against the background (in black). That's fine if this is a limitation of print however I would have preferred to have been notified of this prior to approving the proof. Even a general warning on artwork submission would suffice. Outside of this instance all of my experience with any StickerMule project/service has been excellent!