In plenty of countries you can only enter if you can show a flight within your visa time out of the country. There is the service flyonward.com. But it takes some days to get your ticket and it cost 10$.
Do you have any better proposals?
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Really that is exactly what that service is for. It is cheap and offers people a way to show proof of travel for their return when they do not have one.
The easiest way not to need that service is to actually have a proof of return such as: