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?