Two years ago I ran an ad for one of my novels on BookBub. I set a budged of $5/day and it ran with no problems. So on June 15 of this year I decided to create a second ad for my recently released novel. I set an ad budget of $10/day.
But within a day or so I received charges from BookBub of $750, (since increased to $760), which had been automatically charged to my PayPal account. I immediately checked the data for the ad and found that my budget of $10/day had been increased to an insane $10,000/day.
I contacted BookBub about the error. Their response was, since 'I' had set the budget at $10,000/day, I was stuck with the charges and they were not going to do anything about it, nor refund any of the charges.
I don't know how my budget got changed, only that I did not do it. I can only conclude that there is a problem in BookBub's ad-creation program which they refuse to acknowledge.
Needless to say, I will not be using BookBub to run ads on. (On a side note, I got exactly one sale for my $760. I did much better with a one-week ad on Facebook for a tenth of the cost.)