Hey Christine,
I had the same alarming question. I talked to them on the phone and asked them about that. They assured me they would not take any extra money other than the initial fee. Now, whether they try and pull something in the future when it's time to pay their taxes, (which I also asked about) who knows...
Coincidentally, I have been looking at this new company that's been coming across my social media, called "Clearbanc". I talked to them on the phone today and read all the FAQ (I havent yet, read up on reviews yet). This company sounds much more like something I'd be willing to try, and I probably will in the near future. The way it works vs Payfully:
They offer you a "loan" based on your Airbnb/Homeaway longevity, status and future bookings. Then they base their fee off that...So I asked them on the phone "How much is the fee for $5000 loan?" And they replied, depedning on your Airbnb status (which they check into after you link them to your account). So for $5K he said it would be anywhere from 15-20% fee added into the loan payback amount.
How Clearbanc works:
You ask for a loan. They approve an amount. You get the money in 2-5 days. THEN, as you get your airbnb payouts, they take a cut, depending on the size of the loan, but they will never take more than 50% of your airbnb payout. Which I think is fantastic as Payfull takes the whole thing and you are left feeling broke and cant pay your mortgage or whatever... So, for this reason I would never use Payfully again and am definitely looking to use this company after I get back into town from a trip.
Hope this helps, but make sure you do your own research as I havent used them yet.
Kristen