Purchasing Bits with PayPal works the same way as with other payment options. Simply click on the "Get Bits" button above enabled video streams and select a bundle of Bits. You'll now see the PayPal option along with the others, like Amazon. If you've used other payment systems in the past, you can click on "Change payment service" and choose PayPal as your funding source. Plus, according to Twitch, "PayPal may be an option when purchasing bits on iOS and Android devices if you reside in a country where Apple or Google offer PayPal for in-app purchases."