A Day In May Design is a lovely invitation studio, based out of San Francisco. I was just perusing their site and have fallen in love with the soft elegance of their work...
One of the coolest things about this studio, is that you can do a lot of the work online. From choosing your invitation style and ink color, to adding envelope liners and even rounded edging. Once you have come up with your design, you just phone in your order. Easy as pie. To make the deal even sweeter, you can place a sample order to see what type of paper you want to use.
A tip...make sure to zoom in on their portfolio so that you can see the quality letterpress that they are producing.
Whatever invitation studio you end up using, Martha has some great tips for designing and customizing your invitation suite. Take a look at this article for some good advice on paper type, paper weight, typestyles, etc. The whole process can be a little overwhelming, (from letterpress to engraving to thermography...from cotton paper, to tissue liners, to inner and outer envelopes) and the Martha Stewart article will really come in handy!