You have a couple of options:
What is done on my spree 1-3-stable site is that the
add to cart
button is calling an ajax method that populates the order, thus no redirect is necessary.remove the
redirect_to cart_path
from you method and redirect back to the page you came from.
Edit: AJAX example in coffee
data = $("#order_form").serialize();
#add additional info if needed
data= data + '&size=' + user_selected_size
$.ajax
type: 'POST'
url: $("#order_form").attr("action")
data: data
success: (result) ->
#your success method for handeling what happens after the cart is populated