If you just want something simple I would use ms word with the envelop/label/postcard layout option it has.
Print the labels on regular copy paper, cut them apart and staple them to the bags. I know they also make a sticky backed label paper meant to be used in printers so you could stick the labels on instead.
If you want to get fancier with a logo or something there is a program called paint.net . It's freeware. Supposedly it was some college project overseen by microsoft with the intention of combining the capability of photoshop with the user friendliness of ms paint or some such... it takes a little time to explore it and figure it out but it's fairly capable and much easier to use than something like Gimp (Which is also a free photo editing software).