The only payment button I had available was a fixed amount.
I know what you mean regarding PayPal and slow payers but if it gets a certain few customers paying then it's better than dropping them. It's down to what their comfortable with whether it's cash on the day (only), posting a cheque, bank transfer or PayPal.
I've seen a look of horror from customers when I spoke about gocardless as being the way forward for many professionals. A lot of this work is new to my round having bought it one year ago I've only been cleaning it myself since August/September so it's still a work in progress.