You don't have to be evil to do CSS here.

J W Wells & Co. in Tom Holt's books

The Portable Door
In Your Dreams
Earth, Air, Fire and Custard
You Don't Have to Be Evil to Work Here, But It Helps
The Better Mousetrap
May Contain Traces of Magic

I think more or less I read in a relatively fast pace.

In this 4th book,  'You Don't Have to Be Evil to Work Here, But It Helps'. The love philter appears again. This time they have come up with one phrase. A phrase that describe the consequences of being affected by that potion.

The phrase is simply, 'Till death do us/them apart'. Yea. Once two person drank the love philter, there is no 'cure' or a way to stop the effect of the love potion until death.

So there are still 'exits' to this love potion. One, by dying and somehow come back. Totally plausible if one has the Portable Door as mentioned in the previous books.  It also seems that if the person had true love before drinking the love potion, the potion is ineffective.

The main character from the previous books in the series have apparently went to 'happily ever after' as this book has a new main character whose name is Colin. But there are still references to Carpenter in this book.

This book talked about the 'selling-soul' thing in J W Wells. Apparently, its for real. The company will seemingly get in touch with the 'company' Hell runs. There will even be a formal paper contract to sign. The soul of a human (to hell) in exchange with some magical work provided by J W Wells.

Mr Dao appeared in the story again.

There is also this apple tree. The apple from this tree when consumed, makes the person immortal. Get to live for an infinity amount of time.

The soul exchange contract in the story got 'messed up' in the end. The person who is giving away her soul got to live forever, so J W Wells had to fufill their promises for an infinite period of time also.