The built-in Quantity field for Single Products should be available for Conditional Logic. User Defined Price products should also be available for Conditional Logic.
We have this on our list of requested features already for possible inclusion in a future version. Thank you.
Thank you, Chris, for considering this feature request. We run into many situations where this would be incredibly useful. For example, “If Quantity of X is not empty (or > 0), then display HTML box explaining something and display text box for additional user input” – right now, we cannot hide/show HTML or other fields, based on the Quantity of a product field. Another situation would be to alter shipping based on the quantity of an item being ordered.