This past weekend, I finally pulled the trigger on having the Instagram prints done through Persnickety Prints, which I'd heard good things about, and was excited to get the prints in the mail earlier in the week:
I chose to do the 3x3 cardstock prints with 1/8" white border. I printed between the 15-99 quantity, so they were 45 cents each. I also stuck with the standard cardstock that it comes with, and USPS shipping was $5.00.
I like the fact that I can order square prints, and the cardstock is a nice weight, but it's a bit glossier of a finish than I think I like, and the photo quality seems to be lower than what I got out of the photos I printed at Costco. I used the Instagram sync feature offered by Persnickety Prints, so it's possible that using that contributed to the printed photo quality.
Overall, I'm satisfied with the order, and will order my Instagram prints for the remainder of the year the same way for consistency. I might try to actually upload pictures when printing for my next project, as well as possibly try a different paper. Poking around the site, there are a few other things that look neat to have printed: 5x5 Mini Book, Insta Posters, and 3x4 Retro Prints.