Granted this is a totally inadequate solution, but in all cases where I've made an online order from a reputable seller and wanted to cancel it before it shipped, I have been able to call the business over the phone, provide them with the invoice number, and successfully cancel the order. I've only had problems in cases where "I should have known better than to order on that site" to begin with.

I'm perfectly fine calling someone on the phone, but many other millennials strongly dislike phone conversations.

Every reputable seller also has email. In Amazon's case you don't even need that, you just need to click a button.

  In Amazon's case you don't even need that, you just need 
  to click a button.

Yes, that was the point of the thread. On Amazon's site, order cancellations are automated and can be done on the website, while for many other online stores you have to contact a human to do it manually.

"Due to unusually high call volume..."