A solution can be found in the function
woocommerce_add_to_cart_action. We have to chain two filters. In the first, we check for the product ID, if it's one of a list we add the redirect.
add_action( 'woocommerce_add_to_cart_validation', 'check_ids_wpse_119466', 10, 3 );
function check_ids_wpse_119466( $true, $product_id, $quantity )
# Not our products, bail out
# Adjust the products IDs
if( !in_array( $product_id, array( 1306,1303 ) ) ) // <------ ADJUST LIST
add_filter( 'add_to_cart_redirect', 'redirect_wpse_119466' );
add_filter( 'allowed_redirect_hosts', 'whitelist_wpse_119466' );
function redirect_wpse_119466( $url )
function whitelist_wpse_119466( $hosts )
$hosts = 'paypal.com';
There's a third filter that we need to add, and it tells WordPress that PayPal is a safe host to redirect.
Yes, the list of products has to be made by hand. Or add an option somewhere in the theme, in a plugin or even WordPress settings to put this values (with a multiple selection field).
Related: Where do I put the code snippets I found here or somewhere else on the web?