A deck of cards (Bicycle brand) in a small bubble envelope weighs 4 ounces. The first class mail parcel rate would be $1.64 (each additional ounce is 17 cents, if your cards are heavier than mine). First class parcel rate is the most expensive of the three first class mail rates under the new system. The parcel rate applies because the package is not flexible.

If you send it via Priority Mail, it is $4.60. They take about the same amount of time to arrive.

A lot of eBay sellers like to use priority mail because they get free shipping supplies from the post office. As you can see, the post office makes a good profit on the postage difference, and the buyer usually gets stuck with the bill.