I just ordered our save-the-date postcards from Overnight Prints to arrive by next Friday for just under $20. I went with them over VistaPrint because OP's interface is friendlier and gives you more space for your design on a 4x6" postcard. They have about equivalent review ratings. VP was a bit less hassle to figure out the cheapest price I could get 50-100 postcards printed with them, as they have just a free postcards offer going on, but their shipping is pretty outrageous.
OP, on the other hand, had quite a few different promo codes/links that I could enter in, which changed the final amount by a few dollars here and there. So, word of advice: mess around with the promo codes if you're going to buy from one of these online companies. Note down any that you see in the AdWords ads on Google search results too, because I could've sworn I saw one that said 100 free postcards, but then I couldn't find that code again.
I will report back with my thoughts on the quality of their printing (I chose the "value" digital press since we could get a run of just 50 instead of 100 that way) and the final final design once they arrive and get sent out.