Can you tell me if one can mix "synthetic" and "regular" R134a? Or, is it just that the stuff labeled "synthetic" (which is more expensive) is a marketing ploy?

Marketing ploy. R-134a is already 'synthetic' but it was also supposed to be temporary.
I wonder what the industry will engineer next?

R-134a is a specification--they are the same. The word "synthetic" carries a premium these days. Other marketers use the word synthetic in selling brake fluid.