Ubercart

Ubercart ships with some modules that define gateways for processing credit card payments, and if you're using one of those, my guess (I didn't write it, just a guess) is that when you use a gateway, for processing payments, Ubercart does not require a payment method. It might be relevant to know whether you are using (and already have this running) PayPal Website Payments Standard, or what. Maybe Ubercart just redirects to Paypal for payment submission, for example. We're talking about a special payments method, because you want to just mark the payment made, and have the transaction data stored, and there's a receipt. I mean, receipts are visible, right? An order has a payment tab, you want that receipt generated immediately. Now, I'm not looking at the interface, but isn't there a way for you as the administrator, to browse to an order’s Payment tab, and enter a payment--there's a form on that page? That form works, without calling the uc_payment_process_payment function, which is to say, without actually processing a payment through a payment gateway. So there's another function being used by the code there, that just enters a payment, inserts a row into a table. And that is the function that you want to use. Now, you probably have a headache--off the cuff, I'm thinking specifically, that you look in ubercart payment uc_credit uc_credit.module at the function that implements hook_uc_order(). That's function uc_credit_uc_order(). There's a 'submit' case. This is where the attempt to process a CC payment is made. Which is to say, that uc_payment_process_payment() is called. On line 252:

 251     $order->payment_details = uc_credit_cache('load');
 252      $pass = uc_payment_process_payment('credit', $order->order_id, $order->order_total, $data, TRUE, NULL, FALSE);

You don't probably want to mess w/this, lightly, but for your requirement, it seems to me like something needs to change before that line 252, to make it conditional, as in, if the price is zero, then don't call uc_payment_process_payment(). Rather than change this function(uc_credit_uc_order), it would be better to leave it, and avoid calling it. But, I'll say, hypothetically, that you could just check the order_total, and if it's zero, then just set $pass to TRUE and skip line 252. Might work. I won't write an if-statement for this, unless you beg me to, as it would be better, as I say, to leave this file alone, but I still think you have to change a line or two of PHP somewhere, if not here (you weren't planning to write a custom module, is my guess). Also, you can't just set $pass to TRUE, you need to call some other function, which I could figure out--a function that will update the order status and everything, this is why I was talking about the admin interface--I know or suspect? there's a form there for you to enter a payment, and that would be the way that I would figure out what is the right function.
Which, ok, I've looked in ubercart payment uc_payment uc_payment.module, here is line 365:

365 function uc_payment_process_payment($method, $order_id, $amount, $data = NULL, $default = FALSE, $selected = NULL, $redirect = TRUE) {

and the next function is on line 470:

470 function uc_payment_enter($order_id, $method, $amount, $uid = 0, $data = NULL, $comment = '') {

Now, the first function does this: 'Processes a payment through an enabled payment gateway'
And, the second function does this: 'Enters a payment for an order.'

That's the other function I'm referring to. Let me suggest, that you are not going to touch anything--let's talk further. But, what I've told you so far, is still looking promising 2 me.

Anyways, as to disabling the form that collects the user's shipping address, for this I don't think you'll need to pop the hood. Try here: http://www.ubercart.org/docs/user/3345/selling_files