The PayPal buy now button image in the "Buy Domain" tab of Subsite Dashboard->Tools->Domain Mapping is called via http and not https, which is causing a mixed content error. Looking at the code, it seems like the protocol is hardcoded into the plugin.
To replicate, follow these steps:
1. Set up eNom with Domain Mapping in an https site.
2. Use Pro Sites Checkout for eNom.
3. Go to Subsite Dashboard->Tools->Domain Mapping->Buy Domain tab and check out the console.