this error will occur if the recordset used for the eCart button is empty when the add to cart button is pressed.
I see that your WADAproducts recordset is set to use a sizeChoice and colourChoice posted form element, but your add to cart button form does not contain these form variables, this is causing the recordset to return an empty data set when the add to cart button is pressed.
The empty data set is causing the undefined variable error.