Question:

What service can I use to find inexpensive flights with flexible travel dates (+-N days) and in-flight wifi?

kayak, my typical goto source for finding tickets, allows me to use flexible dates to minimize my travel costs. I then buy the ticket direct from the airline. It doesn't have wifi information, AFAIK.

routehappy, the site hyped by OP, allows you to sort by happiness and select on features like wifi. However, it doesn't appear to allow flexible travel dates, nor can you sort by price, or even see the price (!?).

What travel aggregator should I use?