I'm posting this here since the only help I could find on this topic came from a rather vague YouTube video that moves too fast to see.
When Amazon SES gives you your TXT and CNAME records, you don't enter the part that is your domain on namecheap.
The value stays the same but the TXT Name should just be "_amazonses" No need to add the domain name.
Same thing for the DKIM records. Simply drop your domain name from the host entry.