HI,
If you want to consider using batch class, you can make use of below code to get started

Note that I am assuming you would have Payment_Due_Date__c, Payment_Status__c as custom fields in your org

public class PaymentReminderBatch implements Database.Batchable<SObject>, Database.AllowsCallouts {
    
    public void execute(Database.BatchableContext context, List<SObject> scope) {
        List<Contact> contactsToUpdate = new List<Contact>();
        List<Messaging.SingleEmailMessage> emailsToSend = new List<Messaging.SingleEmailMessage>();
        Date currentDate = Date.today();
        
        for (Contact contact : [SELECT Id, Name, Email, Payment_Due_Date__c, Payment_Status__c FROM Contact WHERE Payment_Due_Date__c >= :currentDate AND Payment_Status__c = 'Due']) {
            // Check if payment is due within 10 days
            if (contact.Payment_Due_Date__c.addDays(-10) <= currentDate) {
                Messaging.SingleEmailMessage email = new Messaging.SingleEmailMessage();
                email.setToAddresses(new List<String>{contact.Email});
                email.setSubject('Payment Reminder for ' + contact.Name);
                email.setPlainTextBody('Dear ' + contact.Name + ',\n\nThis is a reminder that your payment is due on ' + contact.Payment_Due_Date__c.format() + '. Please make the payment as soon as possible to avoid any late fees or penalties.\n\nBest regards,\nPayment Reminder Team');
                emailsToSend.add(email);
                
                // Check if payment is not made within 5 days of due date
                if (contact.Payment_Due_Date__c.addDays(-5) <= currentDate) {
                    emailsToSend.add(email);
                }
            }
            
            contact.Payment_Status__c = 'Reminder Sent';
            contactsToUpdate.add(contact);
        }
        
        update contactsToUpdate;
        
        if (!emailsToSend.isEmpty()) {
            Messaging.sendEmail(emailsToSend);
        }
    }
    
    public Database.QueryLocator start(Database.BatchableContext context) {
        return Database.getQueryLocator([SELECT Id, Name, Email, Payment_Due_Date__c, Payment_Status__c FROM Contact WHERE Payment_Due_Date__c >= :Date.today() AND Payment_Status__c = 'Due']);
    }
    
    public void finish(Database.BatchableContext context) {
        // Any post-processing code goes here
    }
}

If this information helps, please mark the answer as best. Thank you