I'm still inside the trial period but I was nevertheless able to successfully map my Squarespace site to my domain.
However, all mail to @mydomain.com is now bouncing. As a layer of complication, I run all @mydomain.com email through Gmail, so my DNS records include Google's MX records, but my email only started bouncing after I made the switch below.
My A records look like this:
NAME: @.mydomain.com | TYPE:A | [squarespace IP address]
NAME: *.mydomain.com | TYPE:A | [mydomain.com IP address]
NAME: ftp.mydomain.com | TYPE:A | [mydomain.com IP address]
NAME: mail.mydomain.com | TYPE:A | [mydomain.com IP address]
Obviously, I have the two Squarespace CNAME records in there as well.
On the Squarespace page to check whether your zone records are properly set up, I recall there were three additional @ A records, all backups. They disappeared once the first one was recognized. I don't think that has anything to do with my problem either, but I'm mentioning for the sake of completeness.
So, is there anything I'm doing wrong here? Any help would be appreciated.