Hey, I got another great idea. I was organizing my photos when I suddenly remembered your question. I know I already gave an answer to this one, but I'd like to give you more options.
Here goes...
Choose the best photo you have of them as a couple (or you can ask them one, but don't tell them the real reason why you're asking to keep it as a surprise) and have it scanned and digitally blown up, like 16" by 20" or 14" by 17". Add a little (remember, just a few words!) message at the bottom and have it framed.
I am so sure they will love this, too. So you decide: the tea or the photo? If I were the recipient, I would love to have them both!
Let me share my idea of a perfect wedding gift for your friends - the Tree of Love! The tree would symbolize their union, and it would constantly remind them to take care of their tree/relationship. You can choose any kind of tree or you can go for the apple tree, which is also known as the tree of love in ancient times.