Just in case you weren’t aware, if you’re on version 4.0 of the admin you’ve got a generic payment page! The address is at https://www.YOURSITE.COM/payment/ So, for example, mine is at: https://www.mountainphotography.com/payment/
This comes in handy when you need to receive money from a customer for something that doesn’t have a set price on your website and thus can’t be put into the shopping cart. Most commonly this is useful for image licensing, which almost always involves a variety of pricing numbers depending on the client and the image usage. You can send the customer an invoice along with a link to your generic payment page, making it easier for everyone involved!