I recently used Ecwid in a Modulus template. I used it for online event registration & payment. It was very easy to set up configure & launch! I will be using it in future instead of Virtuemart.
I used Modulus, it was not designed for Ecwid but it doesn't need to be. Halcyon is designed for Ecwid.
You can use any RT temp-late just be sure to install the RT Ecwid component to any template to very easily incorporate it into a RT template.
The entry form was very easy. All the custom fields are just product options, just like if you are ordering a t-shirt size, I added options to select class instead of colour, size, etc...
I love Ecwid, it is very simple to use. It is free (I think up to 100 products) I used the free version. I was highly impressed at the speed I could get the store up & running & how easy it was to configure the payment processing. They have all that figured out, you just select the payment & shipping methods you want to use & enter a few parameters & you are done. I don't use the shipping but looking at how it is set up it looks like that is just as easy to configure also.