Hello fellow vinyl graphics neighbor, Check out my site.
All of your points are easily doable and I dont rate my site as slow.
Download the trial version for a few weeks and use it.
CS-Cart would have no problems doing the things you want.
There are some things that you have to do to make the cart faster than just right out of the “box”, but you can definitely get it moving along pretty good.
USPS is no problem with CS-Cart. It takes a little bit to learn especially if you want to use both flat rate shipping and weight based shipping, but there are tons of threads about this and if you can’t find what you are looking for, most of us are happy to help.
I use Paypal standard as well as Authorize.net and everything works great. The redirection back to the cart is actually set up through Paypal. While I haven’t done it yet, you could make a “Thank You” page and then have Paypal redirect the customer back to that.
The product options work great with CS-Cart. You can add cost variables no problem so you should be fine there.
Man, don’t we all know it about the money thing. Just do what John recommended and download the trial and test it out for a couple of weeks.
There is a learning curve with CS-Cart, but once you figure it out you’ll see it works really well.