How can user select attributes for a product when clicking on an add-to-cart link? I'm trying to work out the best way of displaying an attribute selection form when a user clicks on an add to cart link (not on a product node). Ideally display a form in a shadowbox with attribute selection and quantity fields and a submit button. I feel like this option must exist somewhere, but I haven't found it.

Any pointers for me?
Very old one but why not answer?

Use Ubercart Cart LInks module (included in Ubercart Core). I think it allows links that operates on the cart, like empty cart or add a product what I guess includes product attributes.

Here it's explained, but is very old page, check module documentation.

http://www.ubercart.org/node/1427