I don't think you need anything special to print that stuff for COMC. In terms of labels/postage I've become a big fan of thermal printers (I have a Dymo LabelWriter 450 Turbo). That's about all I print these days so I no longer have a standard printer. If I was to get one I'd go with a reasonably priced laser because inkjet toner doesn't last long and is so expensive.
If you're only looking at shipping cards to them a couple times you might just want to see about printing at a public library or FedEx office or something.