Truly is amazing how some who have the money have to be begged to pay their debts.
Helping my son with his accounting we find the old pensioners (he does give special discounts to) pay almost immediately, businesses seem to think because he is a small business they have the right not to pay, yes tedious, almost like extracting teeth, however when work is complete you expect payment.
Over-payments I normally advise the person that they have a credit on his books to be deducted from next accounting bill, unless the amount is large in which case we return immediately, each transaction movement costs over here.
Ouch, it costs for transactions over there? Luckily, here it is free transactions for EU transactions. It is a pain as other people would expect to be paid on time and in full... somehow, it doesn't seem to work the other way around!