Venmo used to have a flat fee of $0.25 then they changed it to 1%. I'm guessing for most people, it would never be a problem since most people are usually only sending small amounts to eachother on Venmo (paying someone back for lunch or splitting utilities) but now Paypal gets a larger cut of people doing instant transfer of things like rent payments.
I think most of the % fee-based "instant withdrawals" are done without understanding the effective APR. "It's only 1%!"