You'd think that after 10 years of e-commerce, that shopping carts would be a solved problem. I just sent this feedback to spreadshirt.com.
But it gets better. I ordered 5 shirts this time, looked for an "update" button and didn't find one, and clicked to continue the order. Only on the last page of the order process did I notice that the quantity was still at 1.
So I clicked on my shopping cart to back up the process and fix it. That showed me the item I was trying to order, and I clicked to proceed. But you were actually showing me the last page of the process again, so clicking committed the order... for one shirt.
Finally, heading to the contact page, I get to enter a captcha (even though I'm already logged in) and type in a little tiny textbox to send you feedback, because you don't want to have to deal with the horrors of actually publishing an email address, even though you demand mine as part of the order process.