Learn English – Using a comma after the word “enclosed” in a sentence

No! You can not put a comma before "as a token of my gratitude."

My guess is that as the only cash enclosed is the cash offered as a gift, the information is non crucial, making the phrase non-restrictive clause, and therefore comma would be required.

The phrase is a restrictive clause even in your context. "as a token of my gratitude" depends on "Please accept the cash enclosed herein." You cannot offset the first sentence by a comma.

What makes it dependent? The word "as" makes it dependent. It is used as an adverb, not conjunction.

You can say it like the following ways:

• Please accept the cash enclosed herein, a token of my gratitude.
  
• Please accept the cash enclosed herein—a token of my gratitude.
  
• ¹As a token of my gratitude, please accept the cash enclosed herein.

_{1. When you start or begin a question with a dependent clause, you put a comma after it.}

Above mentioned examples are correct. As "For all b ∈ B" and "In most cases" are the introductory clauses so they must have comma. For more explanation refer this website.