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.
This is overly pessimistic. There's two tricks to enterprise sales, at least in my experience.
First, have a personal relationship with the buyer. Not the purchasing department--the buyer. By which I mean the person with the authority to tell Purchasing to pay you, not the person or people who are going to use your product.
By "personal relationship," I mean direct interactions where you've taken the time to understand what the buyer wants, why they want it, and why they think you can provide it; and where you've demonstrated trustworthiness, an ability to deliver, and interest in the buyer's success.
For an early-stage startup, by the way, this is the founders' job, not a "enterprise sales" person or staff. As a founder, you have credibility and the ability to establish that personal relationship that a salesperson cannot match.
Second, work through the red tape before you deliver your product or service. If you do this while the buyer is still eager to have what you provide, she'll be motivated to resolve the inevitable roadblocks that appear. If you wait, then she'll be less motivated, and that's where the six-month delays come from. Not from a desire to screw you, but because it's no longer urgent.
In some companies, a PO # is sufficient--think of it like a credit card number you can charge against. However, I also like to get a deposit up-front. Once a company has paid you once, the wheels are greased for them to pay you again. I've never had problems collecting on an enterprise invoice when I was paid a deposit.
This opinion is based on selling five- and six-figure gigs as a consultant over the last 12 years. I've lost money three times: once when dealing with a small-time shop (but we renegotiated and I kept the deposit and cancelled the rest of the gig); another time when a huge company refused to reimburse me the first-class airfare we had agreed upon (I gave up trying on that one); and a small recent gig for which I waived my normal deposit requirement (and now I regret it).
Product sales have differences, I'm sure, but I'm willing to bet that the fundamentals are the same. Have a personal relationship with the buyer, and get them to cut the red tape for you before you deliver.
Do this and you'll have the ability to collect far more than $15,000 per year.
But if you want to sell something for $15,000/year, it's not nearly as important.