This is definitely something that I think has been overlooked in this thread.
If you're dealing with serious money (to the point where an individual is unlikely to be willing and able to spend that kind of money) the odds of even being able to use a credit card are slim to none. Enterprise companies do not generally use credit cards for large purchases. You're better off making sure you can handle POs and ACH (which for the life of me I couldn't tell you how to do outside of "try Dwolla").
I'll just throw in a quick plug for WePay here, as you're absolutely correct when dealing with high-dollar invoices and POs. Credit card fees get nasty at that level, and the chargeback risk gets a little hard to stomach. From the perspective of the merchant, we make it just as easy to get paid by ACH as by credit card (plus we offer free invoicing tools, and have other invoicing servies that use your WePay account over our API)
In practice it's a tiny bit more effort for the payer the first time around (there's no ACH equivalent to a credit card authorization), and after that everything is basically one-click.
Having worked with numerous ACH gateways... it's really something you're better off avoiding if you can help it. Never mind the painfully-long merchant agreements, there also tend to be weird limits on your volume, cryptic SEC codes to deal with, and... ugh, the reconciliation processes.
If you're dealing with serious money (to the point where an individual is unlikely to be willing and able to spend that kind of money) the odds of even being able to use a credit card are slim to none. Enterprise companies do not generally use credit cards for large purchases. You're better off making sure you can handle POs and ACH (which for the life of me I couldn't tell you how to do outside of "try Dwolla").