Form 709 may or may not delivered, will I be penalized?

I would like to ask that after my 709gift tax return filed ,  certified mail was mailed on 4/6/2024, the tracking of usps arrived at the MO state facility on 4/9/2024 and has been displayed in transit and delayed since then. It's 5/3/2024 now, and it's still the same in transit. . I'm worried about losing it. Asked the usps, said it probably arrived and forgot to scan it. What should I do, please? I'm afraid the irs won't get it and I'll be penalized a lot. Because my father gifted me the house in oct ,2023 , the amount is quite large of $900,000. Should I send form 709again? But it may be doubled and be penalized for late filing?  Still worried.