Another option is LNURL Zap where you can create an LNURL for any Nostr event and then put the QR code on your site. An anonymous zap is sent whenever someone pays the LNURL. 

https://lnurlzap.com 

 Check out this plugin too
https://wordpress.com/plugins/postr-for-nostr 

 Took me a minute but I think @Joel 🇨🇭 is the author 🤙