I don't understand what the benefit of using sites like Onward Ticket is vs. just buying a fully refundable return ticket and then cancelling it after you arrive in country. I get that Onward Ticket "costs" less money up front and maybe makes sense if you have no credit limit or zero disposable income to spare, but whatever money you give them (even if it's only $16) is lost and you never get it back. Just buy a refundable ticket directly from the airline and at the end of the day you pay nothing at all.
I do this with Delta Airlines quite often while traveling internationally - buy a fully refundable ticket to ATL or JFK and put in on a credit card, print the ticket off in case anyone from immigration asks for it, and then once I'm cleared by immigration I cancel the ticket and the money's returned to my credit card in 3-5 days at most. In my mind it also looks less sketchy in the unlikely event immigration decides to do any digging - I'm sure they have the means to figure out you're using a service like Onward Ticket if they really wanted to.
The other upside of buying a fully refundable ticket directly from the airlines is you don't have to worry about "timing it right." For example, if you buy a fake reservation through Onward Ticket, or if you buy a nonrefundable ticket from a U.S. airline and try to take advantage of their 24-hour risk-free cancellation period, then you essentially have to wait until the last minute to finalize your trip planning, and (in the case of a nonrefundable ticket) there's also the possibility of forgetting to cancel your ticket in time and having your money tied up with airline e-credits.
I'd rather just buy a refundable ticket a few weeks in advance and have the peace of mind of knowing my trip planning is complete and I can get a full refund on demand, rather than waiting until the last 24 to 48 hours to tie up loose ends and potentially screwing something up in the process.