Check the design editor and the shopping cart to see if the back is blank or if there is a pattern. If there is a pattern, it cannot be removed. Please note that if there is a full-page picture frame that is not filled with a photo, the background will print as seen in the shopping cart preview.
Also, please note that if you choose a glossy layer, this applies only on the front side for flat cards, and on the front + backside for folded cards, so you can always write on the back (flat cards) or inside (folded cards).