I have occasionally accepted bank transfer at the location when the Client has decided to 'upgrade' during the booking (owo or extra time)

Everything stops the moment I am asked as we fumble around for our respective phones and complete the transaction. The Client has always already paid either deposit or full fee transfer before i leave home, so already has all the details and it takes less than a minute.

One thing I never do is give the service and trust him to pay afterwards, no matter if I have known him 10 minutes or 10 years. Ever.   

Yes I've been confronted with incorrect fees both at start and end, by new and regular clients. If I accept at the end I have to accept it may be wrong, so much easier to sort this out at the start.

Also once had someone tell me they were tipping me by paying the equivalent of a longer than booked booking, yet only gave me the exact amount for the time booking. This was fine no problem I assumed I had misheard or miscounted. He then produced the same fee for a longer second booking, because I had taken and counted upfront I was able to show him it was wrong, and that I had not been tipped as he thought the first time. It was all ok and I seem to remember he called the second booking short in recognition that I had actually also over stayed for the first time (my own choice I had stayed between the actual time and the time he thought he had tipped into).

Point of this is always best to get the money upfront, at the end can be too late (as I found out about a year ago with someone I had gone out of my way for(and he got a flea in his ear about the subsequent poor form with the cash)).

As for bank transfers with transfers I know I have it means I don't have to go to the bank, but I have to see it at my end on my gadget before I do anything and am very clear it's cash at appointment time.