Besides processing credit card information in a secure way, a shopping cart is essentially a form maker. The "Order" button allows a user to take an action, which adds bits and pieces of data into a form. Once the information is securely packaged, including the credit card information, then the data is passed through to me online. As the t-shirt sales person I read the information and either fill the order, modify the order or delete the order depending on the inventory and validity of the credit card processor response. It is helpful to have formatted data in a form that allows me to do my job like a checklist instead of like a crazed maniac trying to decipher emails filled with bad artwork and vague references to t-shirts.
The problem with a shopping cart is that if you project all of the ways in which a person can answer all of the questions that need to be answered, then most of the time the person is gone before they finish submitting the t-shirt shopping cart form. Here are two examples of mediocre forms linked to t-shirt databases. BlankTshirt.com and NCShirts.com. These forms work and they are tied to a larger database of information, products and pricing. My personal opinion is that if someone simply emails me back the basic information for a potential t-shirt order, then I can confirm the order and process it without a fancy cart.
This is an example of what I typically need to process a t-shirt order:
Brand, Style Number, Color, Quantity for each size.
Repeat until complete.
Name, address, zip code, phone number for shipping, email is whoever sent it to me.
I can then make the order and create a link for processing the payment.
A shopping cart will do these functions for me, but at what cost? If I can't keep the information up to date and the formatting of the t-shirt shopping cart is too difficult, then the shopping cart will bog down the entire system. I agree that a shopping cart should take the credit card processing information, but I can arrange that separately or have a shopping cart just for people to sign up and put in their credit card data as a registered customer. Once they are in the system then I can accept formatted emails and process their orders quicker than spending all of my time building a perfect cart.
There are systems for payments that provide a secure environment to process payments, but I don't trust them any more than they trust me, like Paypal or Amazon payments and possibly a bunch of others that I haven't tried. Also there are systems to allow users to sign in through their universal login, like Facebook and so on, but I don't like my activities to be tracked so that I am constantly being hit by advertisements. It's like they are in my brain with these friggin' advertisements these days, Jeez. Stop it already. There isn't an easy answer, but if there way it would simply be send me a properly formatted email as listed above and then I can fill your order or provide a link for you to fill your order in the most efficient and affordable way possible.
The above described process makes me want to go towards Wordpress for the registration system, but Google also has some interface that may allow people to join circles and communicate in a group after registration. With Wordpress there are so many fake users that I don't even have the time to delete them all, much less their comments. With Google I may be able to allow customers to access a spread sheet and fill in their order and we know how much people love spread sheets. Still it is the communication and form that are important here, so for now I am just going to encourage that my website users look through my products and then send me a formatted email, like above, that I can quickly review and respond back with a link to place an order. It's barbaric I know, but this is a default instead of jumping from one cart and proprietary system to another.