It's planned but may take a few months to make it available - payment systems are very complex to set up and integrate. In the mean time we support Stripe.
Is PayPal not supported at all currently? Also did I miss an announcement or there was none (at least yet) that C3 is available for purchase?
Btw, just a note: I'd strongly suggest to change the "Splash Screen" text to "Custom Splash Screen" on the Construct.net Pricing page since it might drive users into thinking that they'll be forced to use a C3 splash screen with every license.