So let's assume you have two additional fields, one for the label text and one for the additional user info. Set your combo-box to commit the selected value immediately and then apply this code as its custom validation script:
var labelField = this.getField("LabelText");
var additionalInfoField = this.getField("AdditionalInfo");
if (event.value=="Collect personally") {
labelField.value = "Please contact us when you're here.";
additionalInfoField.value = "";
additionalInfoField.display = display.hidden;
} else if (event.value=="Courier") {
labelField.value = "Please provide your delivery address:";
additionalInfoField.value = "";
additionalInfoField.display = display.visible;
} else if (event.value=="Post") {
labelField.value = "Please provide your delivery address:";
additionalInfoField.value = "";
additionalInfoField.display = display.visible;
}