Weight: Paper thickness is measured by weight. The higher number you see, in pounds, on the packaging label, the thicker your paper is. This doesn't mean you have to find the thickest paper you can—most card stocks are a great weight for invitations and can hold up to being mailed.
LOCAL PRINTING: When it comes to short run printing, the local option is my favorite option. Local print shops may not be able to match the price of many online vendors, but you get to have more of a hand in the process, and therefore a little more control over the end result. Some will even let you bring in your own paper, and most will let you use their paper cutters for free (sometimes even if you haven't printed anything through them). Unfortunately, not all local shops are created equal—some FedEx Office shops are great, others not so much. You'll need to do your due diligence to find a good match. InkerLinker is also a good place to find amazing, non_FedEx print shops near you.