Hi, John! My family and I LOVE the Design-a-Tee store in Downtown Disney's Marketplace. I'm sure you've probably already seen it, but you can read more about it
here. When we were there last October, Guests were able to design t-shirts with Halloween-themed art, so I am sure that there would be Christmas-themed art for your vacation as well. In fact, I am 99.9% sure there was a button that allowed you to choose "holiday" themes.
If you go early in the day, shirts can be ready in as little as 15 minutes. However, as the day progresses, the wait time can increase. Package delivery is available to Walt Disney World Resort Guests. If you think you want to design a shirt, make this your first stop when visiting Downtown Disney. Then shop or enjoy a meal before picking up your souvenir. Happy planning and have a Magical vacation!