I would say in the long run you should give out the PDF for free and charge for sending out the prints on proper card/paper. Most people are going to pay for something will want it printed properly. Also, I imagine the main advantage would be time saving not cost saving.
There is vanishingly little incentive to the business in giving that much value away for free. It autocommoditizes your offering and lets any idiot with a printer compete with you.
Give the PDF for ~10 guests away for free. This demonstrates that your solution will work, but nobody worth worrying has their invitation need filled by 10 invites.
Charge appropriate to value for PDFing the full guest list. Charge extraordinarily dearly for completely taking one annoying task off the overworked bride's plate and sending the invites yourself.
Ultimately the price of the PDF is going to tend to zero anyway, that's why I said in the long run. If you charge for the PDF someone will charge less and someone else will charge even less. I don't think people will want the PDFs since their printers are not very good + they don't have the right paper + ink is expensive.
The free PR you get from offering the free PDFs will drive sales of the printed versions and providing some free PDFs to low value customers will be much cheaper than Google ad-words and wedding magazine adverts. If you think about it magazines / blogs are going to point to the place you get good invitations for free, ultimately a lot of the people going there will pay for them to be printed and sent out.
If you charge for the PDF someone will charge less and someone else will charge even less.
Let them. Your pathological customers have to go somewhere -- send them to that sucker. If they cut their prices, you raise yours. I promise you you will come out ahead on that exchange. (It even works for bingo cards, which the customer has not spent an entire lifetime envisioning as the ultimate in socially acceptable public demonstrations of lavish consumption.)
It should be possible to partner with someone to do the actual printing and mailing. Ugly PDFs might sell for zero. But gorgeous PDFs will not. Also PDFs by people who actually understand what the customers want might be worth much more.
