Rule #1 of business: Don't make it hard for people to give you money

As consumers, we are drawn to convenience more than almost anything else. Making payment quick and easy won't make a sale for you, but making payment difficult will give the buyer time and an incentive to change their mind. However, the real issue is that it is a bad user experience. To illustrate let me give you a couple of examples.

Why do we have pay at the pump? It's not because it's expensive to have a cashier. All gas stations still have an attendant, it's because it makes the gas buying experience more pleasant for the person buying gasoline. In a day when almost all gas stations are also convenience stores, the business would love for you to come in the building because you are much more likely to spend more money if you are in there with the soda, candy, snacks and milk that you just realized you need. But the user experience trumps this.

Think also about a long checkout line at your favorite big box store. How many times have you abandoned a cart full (or even just a single item) because the line was long and the wait irritated you? Did you really do it to save yourself time? Was there a rational calculus where you decided that coming back at another time to buy the same item would be more efficient, or were you just angry? You were just angry. Don't make your customers angry. 

We can delve a little deeper into this if we want. Think about classical conditioning. If you make the checkout procedure difficult, you are, over the long term, conditioning your customers to avoid it. This is the opposite of what you want to do. You want to make all parts of your product or service positive, but most especially the one most closely connected with the buying decision. When you make the final act of purchasing difficult you are decreasing the positive emotion of your product in a multiplicative way. In other words, the negative feeling when applied elsewhere in the customer experience is not nearly as impactful.