1) Install the watch face from Pebble app store as usual.
2) After 24 hours of free trial use, you will get a message on the Pebble watch.
3) Visit web page indicated on the message (something like kzl.io/sp), and enter the code shown on the watch.
4) Complete the purchase with a PayPal payment.
When you are about to purchase one of the watch faces, the sales system will offer you the bundles that include it.
* The license of a purchased watch face is linked to your Pebble account. Payment is always a one time charge, and you paid only one time the watch face even if you have more than one Pebble watches.