Astraware Sudoku has a single In-App Purchase available - the 'Go Ad-Free' option.
This makes each of your daily games, and any of the anytime games (aka 'Random Puzzle') ad-free.
This is a permanent unlock. If you've previously bought either the ad-free or generator purchases, you will automatically be upgraded to the full ad-free unlock.
If you prefer to play for free, you'll get a short advert (usually 15-30 seconds) before or after each game.