electronic postage?

Does anyone have any experience using electronic postage? We have a
site that needs to send acknowledgement cards periodically using snail
mail. We would like to print the whole card, including postage, all at
once. Then we just have a pile of things to be placed in the mail. I
know there are several electronic postage vendors (endicia.com,
stamps.com, etc), but I haven't seen a gem or plugin that might help
with this. Any ideas?