IMO I would try to goodwill the mortgage servicer. Use this as a wake up call to change your system of payments and checking to see that the payment was drafted out of your bank accounts. Not looking at your bank statement to reconcile at least monthly (and usually more frequently) is a danger for you for any payment you have to make.
If you both use the same checking account, then consider having a separate household account where you deposit your funds for monthly recurring household bills like the mortgage, car payment, utilities etc. Set that HH account up with auto payments and then check it weekly (or daily!) to reconcile.
Also please understand where I am coming from when I say this: when you pay your mortgage on the first of each month you are paying the prior months interest + a small principal payment. So the payment due on Feb 1 (for example) pays January interest + a small amount of principal. The later the payment is made in the month, then more of the payment is going toward interest and less toward principal because it is calculated per diem (per day). You will end up paying tons more interest over the life of the loan if you continue to pay past the due date. When you run an amortization calculator it assumes you pay on time, everytime.
Once you get back on track, you can make up for all the months you paid beyond the due date (this is any day after the first) by paying just a few days early. If your payment is due the first of April (as an example), pay Mar 27th. It may seem like a small thing, but the saved interest adds up. Plus it gets you into good payment habits.
Once you have done this for several/many months ask for a GW delete of the late. Maybe the mortgage servicer will accommodate you.
In my opinion (as harsh as this sounds) it is not appropriate to ask for a GW if you have been paying late every month (around the 14th per your post). Just because the late fee doesn't kick in until the 15th doesn't mean that the servicer doesn't keep record. They do. They have an internal record for each of your payments. Good thing is this: you are learning very early in your mortgage the importance of checking to see that the payment funds actually left your bank account on time. You can overcome this derog. It will just take time.
