I think these days you can buy postage online and even personalize it with photos of your kids of you want. Poke around the USPS pages or search on "online postage" and see what turns up.

I have found that the PayPal/ebay postage is very easy to use, and I don't have to go to the post office. The mailing label contains everything it needs and I have a nice accurate scale in the kitchen to weigh these things. My letter carrier can pick up boxes at the house and they are easily traceable so it isn't like he's carrying an unknown box with stamps on it (that they obsess about and return so you can give it to a postal clerk in person).

SRS