Well I feel silly for not knowing this sooner (and I've been shipping through USPS for almost a year) but I feel its important to share with anyone else who may not know. On this site there is this very important note:
Note: All Package Services parcels mailed with Delivery Confirmation must measure at least 3/4" at the thickest point, or be in a rigid box.
If you are printing your shipping labels as USPS First Class Mail through Paypal it will automatically add Delivery Confirmation (from what I have seen there is no option to remove this). So if you are shipping an item First Class Mail and printing the label through Paypal make sure that it is more than 3/4" thick or, if caught, they will return the package to you or deliver it to the recipient and charge for Priority Shipping--not something you want one of your buyers to have to deal with!
I've been very lucky this past year and not had any trouble with my mouse pads (they weigh 9oz but when packaged are only 1/2" thick), but from what I've seen in the Etsy fora the USPS is cracking down!