i've not really needed to fiddle much with Illy's save for web but i can only assume it's for raster images. if you're issuing these invitations for people to print themselves, give them PDFs.
so if they're printing them out on their own, like at a store like costco or walmart I should give it to them in pdf form?
I agree with Doug.
You may Save As or Print to PDF. If you Save As, you should untick Preserve Illustrator Editing Capabilities, to reduce file size and avoid passing on unecessary information. Depending on the contents, you may consider the resolution requirements for raster effects/parts. Printing to PDF generally leads to smaller file sizes.
Depending on the printer (maybe in more than one sense), you may create the artwork in CMYK Document Color Mode, or RGB. The former is the right choice for printing presses/PostScript printers, the latter for non PostScript printers (where CMYK colours will be converted twice, back and forth).