These are the prices I am seeing for the US:
The Adventure of the Christmas Pudding $6.99 (contains six longer stories, one reviewer said of novella lengths)
The Adventure of the Christmas Pudding, or The Theft of the Royal Ruby
The Mystery of the Spanish Chest
Four and Twenty Blackbirds
The Dream
Greenshaw's Folly
The Under Dog"
Poirot's Early Cases $6.99 (contains eighteen stories)
list from wiki:
1.1 The Affair at the Victory Ball
1.2 The Adventure of the Clapham Cook
1.3 The Cornish Mystery
1.4 The Adventure of Johnnie Waverly
1.5 The Double Clue
1.6 The King of Clubs
1.7 The Lemesurier Inheritance
1.8 The Lost Mine
1.9 The Plymouth Express
1.10 The Chocolate Box
1.11 The Submarine Plans
1.12 The Third Floor Flat
1.13 Double Sin
1.14 The Market Basing Mystery
1.15 Wasp's Nest
1.16 The Veiled Lady
1.17 Problem at Sea
1.18 How Does Your Garden Grow?
For those in the UK, there are different ASIN's and lower prices:
The Adventure of the Christmas Pudding $6.47
Poirot's Early Cases $6.47