On the beach are cheap (compared to tour operators - but the thing is, they are not a tour operator); and I find their website very good for navigating and finding the best deals.
But I would then always just take a note of the details and book it all direct - flights, accommodation, transfers. Job done.
As pp's have said, customer service is non existent. They don't answer their phones.
You are literally paying for nothing. When you "checkout" with them, your holiday is "reserved" whilst they go into the market and try to buy it. If the price has increased during this time, you will have to pay the extra.
Finally, they are not a tour operator. Meaning if something goes wrong on your holiday, you are on your own. They won't help in any way.
And that is basically what you are paying for with Tui / Jet2 and the likes.
So really, you can pay the extra for that peace of mind, or you can do it yourself for cheaper, but you might have an issue if something goes wrong on your holiday.
I have used on the beach before many times and generally it's been fine, but one year there were issues and it was a nightmare.