After a year or two I expect to be dealing with quite large companies who buy a lot of space all over the Internet
Large companies use Paypal or Credit cards?
They often expect to pay for performance(PPC/PPA), or pay at the end of a campaign, send company checks and all sorts of other nonsense.
A lot of this happens because you often end up dealing with media companies who won't pay you until they get paid.
Then, adding insult to injury, the media company will put 3rd party monitoring systems between the actual customer site and your site, which may or may not count all of the hits properly. Expect to always give them 10% more than they ask for since not all visitors will be able to actually get to either the media company or end customer server due to random routing issues and you definitely want to get paid.
Don't forget geo-targeting, US/UK/CA only impressions, and much more.
Those reasons are why I booted all my large advertisers years ago and will never look back.
Not to mention charging very large amounts on CC's will often invoke the banks anti-fraud which stops the transaction. You're also assuming that the person placing the ads even has a CC capable of processing such a large balance in the first place, which means they'll need the CC from their CEO or something.
Finally, you have the very real risk with large amounts on CC's to have one massive chargeback if they didn't like the ads performance and/or your CC processor withholding funds (thousands) to cover potential fraud/chargebacks when you charge very large fees.
Sure, getting $30K checks are nice, especially since a $30K CC charge rarely clears, but 30K headaches dealing with it all aren't fun.
The small-to-medium sized clients will be fine with regular CCs and Paypal.