Printing photos from Lightroom 5?

I currently did a a senior photo shoot. I shoot with a Nikon D3100 and in Raw format, I then imported the files and edit them on Lightroom 5, After doing so I exported them onto my facebook and my website adn they looked perfect. I exported the files for email as well to the customer and when she went to print the photos at wal greens she said they were coming out grainy. She was going to use them as a graduation invite size 11x7.
What did i do wrong in the process. What do I need to change in Lightroom so the pictures won't come out that way?

You need to send your customer the full size image file.