My website is starting to attract some interest with a few business requesting paid links to their websites. At the moment I have no e-commerce facility set up, so in order to get back to these potential customers, I need to set something up asap. My question is, what would be the quickest and cheapest way of setting up a facility where I could charge people for text links, and send them invoices. I don't think I would need any kind of shop as I would only be selling links or banner ads. Obviously, I'm keen to get back to these potential customers asap so any suggestions would be greatly appreciated.