First, Pay it immediately. Then,…
If you’re 30 or more days behind then yes, it will be reported as a late payment, and it will impact your credit score. Anything less than the full payment would be considered late.
You can expect to lose anything between 50-100 points for this, and a record of it will remain on your report for 7-1/2 years. When potential lenders pull your credit report they look for exactly such information.
What you CAN do is try to sweet-talk the loan company and beg them to not report the late payment. Explain to them that you DID mail the second half of the payment on time, that the mistake in the address was not on purpose and that you don’t want to ruin your otherwise clean credit report for this. You may even suggest to mail them the returned envelop as a proof that you intended to make the payment on time.
Most lenders will show good will and will not report it, especially if your record with them is clean.
One last thing – making partial payments is really a bad idea. You open yourself up to all sorts of problems, including the payment being returned because it’s only a partial payment. Just make the full payment before the due date.