I'm sure many here know about the United Airlines FareLock process, but I had never used it until today. What a great tool!
I'm trying to pin down flights for a not-yet-planned summer 2018 trip. With itinerary details up in the air, I'm having difficulty settling on the timing, especially for my return flight. Though I'm pretty sure I missed the bottom of the airfare market here in DC, there is a decent fare available now for non-stops into Munich (only one flight a day) and out of London over a range of dates. Other dates are much, much more expensive for that Munich flight, so I feel like I need to act right away. But I'm uncomfortable locking in dates as of today.
For United flights (I would guess including United Express), you can select the flights you think you want and choose seats--at least in regular economy--then pay just $4.99 to lock in the price for 72 hours. There's also a week-long lock available for a bit more (I think $6.99). That fee is for the entire reservation--round-trip in my case. I assume that a family traveling together would pay just the one fee but haven't tested what happens on a multi-ticket purchase. [See Mark's post below--I may be wrong about the cost of FareLock on multiple tickets for people traveling together.]
After you purchase the ticket three days later, you have the normal 24 hours to cancel for a full refund, though I believe there may be an exception to that cancellation rule if you buy the ticket quite close to the departure date.
I imagine some other airlines have a similar option, but I wouldn't be surprised if you're always limited to doing this on the ticketing airlines' own flights.