Hi Ricardo,
Thanks for your reply. In your example, such a mistake would be the fault of the advertiser, and I'd happily reimburse them the $20 difference if they requested it later. I think such a mistake would be rare. On the other hand, advertisers wanting to buy 2, 3, or 4 months of advertising would be quite common and very valuable to support.
Please consider offering this flexibility and letting the business owners decide which package offerings will best meet their business needs. If you're concerned about the case you described, you could offer a back-end switch to allow webmasters to prevent the purchase of multiple package instances.
Thanks,
Andrew