Spirit Airlines

It is a great airline if you like to pay only for what you get.

First checked bag - $30 during spirit.com booking, $40 online afterwards, $100 at gate.

Check-in - free online, $10 at gate.

A cup of tea + an English muffin. - $4

You can pay extra ($10- 20) to get a good seat. 

A round trip between Boston and Las Vegas with one checked in bag costs $330.58 in 2018.

It is more flexible and economical to book plane tickets and hotel rooms directly from their website than from a travel company.