By asking ..."is there any way you could pop by today?" you are giving them an option to say no. I would not say this. This is your business and you have your policy for payment. You don't have to be rude but could kindly say your wife didn't pay me before she went away and now payment is overdue. I understand this may have been an innocent oversight but payment must be received by the end of today or a daily late fee will be charged and any further charges I incur as a result, will have to be paid before care can resume. It's matter of fact and professional but also showing the family that you mean business. It will set the tone for the future and let them know that they must play by the rules. Otherwise, why do we have contracts if we ourselves don't stick to them.