I'm not sure I understand the purpose of your reply. You must be speaking about your present service. I am very familiar with S3. It is just absurdly expensive. EDIT - I don't doubt that this could be done. I am just questioning how often you get 100 orders at a time. Of course possible. Congradulations, if so. The only thing that will happen in that case, with any service, is that max speed will decrease slightly.
From what I can offer you will be very similar to Shopify's offering, only added some possible customizations. Customization and automated import is not possible with Shopify's default offering. Price is negotiable, but would be similar. From what I read, it wouldn't be for you too important to save another $10-$20 a month, as you are saving probably in the thousands already (with Shopify).
They must be running web servers, and speed will be good. The best that I could offer you would be to run off two mirrored web servers, but would have to calculate that out. I can certianly garuntee whatever your capacity needs are. With Shopify, you have that disadvantage that you will never know what they are doing. Nevertheless, I suspect that their service will be able to meet your needs.