Worried the customer promising to pay won’t do it? Ask them this “Do you want me to let you know once your payment arrives?”. Never fails to sort the wheat from the chaff
Don’t ever reply straight away to an email with them promising to pay. Keep the “thanks” or similar up your sleeve on the off chance the payment doesn’t arrive, and then send it. This is much more powerful reminder than the awkward “hey why didn’t you pay” email
Customer not answering? Hit redial twice. 3 missed calls and they will be worried and call you back
Customer sent a read receipt but didn’t reply to your email? Reply to the read receipt thanking them and asking if you can discuss further
Worried your customer isn’t answering because your number is private? Dial 1832 first and it will show your number