On average for me, when shipping a videogame (which normally is about 4 ounces), First Class shipping is about $1.86 when I buy it online. You can purchase First Class shipping using Paypal by using this link (I have it bookmarked):
Then you can print out the label and either put it in your mail box, or drop it off at the post office (and not have to deal with postal customer service).
Just another tip, I also re-use bubble mailers too.