Growing Herbs for Tea

I started my first culinary herb garden last year using the 12-plant gift kit from Richters and am very pleased with the results. Now I want to plant herbs for tea this year and have a few questions.
I got 3 lavender plants last year. Though they flowered in fall, the plants haven't grown too much more since I got them. Any suggestion? I am very eager to harvest more buds for a lavender bread recipe and for tea.

I am assuming that you are growing the lavender plants in pots indoors. We find that lavender grows slowly over the winter months even in our greenhouses and even when we supply supplementary lighting. Lavender, especially the english types (Lavandula angustifolia), seem to like a winter rest. With the days getting longer and with the light getting more intense our greenhouse plants are starting to grow again. Likely you will find that you plants will begin to show more life as spring takes hold.
Although all lavenders grow well in containers, there is no doubt that lavender likes to have room to grow. Lavender will thrive better if you can find a suitable spot in the garden. A plant in the garden will produce many more flowers than a plant in a container, and flowers is after all the main reason for growing lavender.

I am also thinking about planting peppermint, spearmint (for cooking), roman chamomile together near the deck. Can I put them together in a regular window box? The lemon balm from last year looks great now, but I heard it could be invasive too. Should I transplant it to the window box as well? Will the mint family plants crowd out the chamomile? If possible, please also advise on how to arrange them or what other herbs/flowers to add to the box to make the window box visually appealing as well as functional.

The mints will quickly overtake other herbs in a window box, so I would plant them in separate containers. For serious mint tea lovers a whole windowbox devoted to peppermint and another to spearmint would be a good idea. That way you can count on producing enough fresh or dried mint to make tea on a regular basis.
Chamomile does grow in containers, but because you will use the flowers, you probably wouldn't get enough to make a pot of tea even if you planted a whole window box with just chamomile. If you have the space, consider growing chamomile in the garden. Ten square feet is really the minimum needed to make tea.
Lemon balm can be invasive if it flowers and sets seeds. But in normal garden situations I find that it is easy to keep in check, far easier than mint. Unlike mint, it does not spread by stolons or rhizomes, so it generally stays where you plant it. Any seedlings that come up around it are easily controlled with normal weed cultivation. If your plants are already in the garden then I would leave them there.
Other tea herbs to grow? There are many, but here are some suggestions:
For containers, including window boxes: try lemon verbena, stevia, lemon basil, other mints (again in their own container).
For the garden, try bergamot (especially the red variety), betony and anise-hyssop.

I will wait for your reply before I place order. Thank you very much.