If you are on the dumpster fire of an ecommerce platform known as Volusion, they have released a new cart checkout workflow. I believe it's only available to their upper level plans, but it's worth contacting support and bitching if they haven't offered it to you.
It's still not as intuitive as it could be (guest checkout still sucks), but we saw an immediate bump in conversion rate. It appears to work consistently across all browsers, error checking works, and we don't get the "I can't checkout" phone calls that were eating our call center alive.
tldr: Volusion is still a hot mess, but the new checkout works.