What to do about mad buyer on etsy?

This person bought a tooth fairy letter for her son on January 19th and I shipped it on January 24th (1 day after it was supposed to because of car troubles). I just got this message from the buyer: I'm really getting pretty annoyed that I still don't have this!!! My sons tooth is still hanging in his... show more This person bought a tooth fairy letter for her son on January 19th and I shipped it on January 24th (1 day after it was supposed to because of car troubles). I just got this message from the buyer:

I'm really getting pretty annoyed that I still don't have this!!! My sons tooth is still hanging in his mouth but I really don't want to be pushing time this close! I'm very unsatisfied with this etsy shop and I will be leaving negative feedback!

What should I say? I want to make the customer happy, I also don't want negative feedback.. I only have about 9 feedback, so a negative one wouldn't look too good. The letter is for the child's first lost tooth, so it couldn't be used for another tooth or anything. What should I do?
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