Another great installment
This is another strong entry in your series, which I've enjoyed thoroughly. The only weakness I can perceive--and your writing generally overcomes this pretty handily--is that the situation is fairly static from story to story. Not the characters, certainly, but the reason and situation leading to the tickling are largely the same. Still, I've enjoyed each story, and they've been fairly distinct from one another, so why mess with a winning formula?
What might be interesting for the next story would be a competition angle--either two women or one man and one woman, whether out of a personal, pride-driven grudge or some strange application of Kentaros law--struggling against each other to hold out the longest without laughing, crying out, asking for mercy, or whatever. It might be a way to spin the situation differently. Still, even if your next installment has new characters going through the exact same thing, I'll still read it and like it!
Just my random input.