Thank you for the updated info on this. I will need to check into this a little more to test out some things. In the mean time you should be able to get it to work by adjusting the filter that you have on this recordset. Instead of a URL parameter you should be referencing a form value. The value should be eCart1_1_ID_Add.
I think that this will get things working for you, post back if not to let us know. I will look into this to see if there is a problem with the internal code for the add to cart from rs.