I just used onwardflights.com to “rent” a return ticket from Guatemala and it worked just fine. I flew Spirit Airlines from Denver, connected in Ft. Lauderdale, and then on to Guatemala City. The really promising thing was that the rented ticket they gave me was also from Spirit Airlines and just I provided them with the confirmation number and date of the flight, the agent punched it into her computer, and she handed me my boarding pass without asking any other questions.
I will say this, onwardflights.com looks (and feels) a little janky, but I was willing to take the risk because it’s only $5, they use PayPal so it’s pretty secure, and you can get a flight from anywhere. Right now, with flyownward.com, they only book certain destinations (200 per day) and you can only “rent” whatever they have available for that day. They do not book tickets in Central/South America. It is a much more legit operation, but onwardflights.com got the job done for me in Central America. I will use them again.