Murder in the Mews can also be deleted from the cart. All 4 novellas from that are also in the Complete Poirot and there doesn't seem to be any special introductory material.
ETA: Also
Poirot Investigates can be eliminated, and
The Underdog and Other Stories (unless you really want that original version of
The Submarine Plan which Wikipedia says was rewritten into the included The Incredible Theft).
ETA 2: The following story collections can be skipped if you only want Marple/Poirot shorts and you're not a completionist/have no interest in her supernatural stuff:
Regatta Mystery &: A Glass Darkly is the only unique story. All others are Marple, Poirot, or Pyne and covered by the Complete Stories and the Pyne collection.
Three Blind Mice &: the title story is the only unique one (and apparently never printed in the UK, according to Wikipedia). All others are Marple or Poirot in the Completes.
Double Sin &: 2 unique stories, which are both supernatural. All others are M/P.
Star Over Bethlehem &: religious-themed stories and poetry, if you're not interested in such.