Please help.. one product is out of stock

I am new seller doing FBM, this morning a buyer bought an item but then I found out its out of stock, I have messaged buyer that its out of stock and asked him to cancel the order, but no response.. I guess he has not read or doesn’t want to cancel..

I tried to find online buying the same item but I worried some are not genuine, and its out of stock everywhere..

I don’t want to cancel order as my sales are very few, so even if I cancel once this time, my rate must be > 2.5% that Amazon allows and worry my account will be deactivated…

What can I do now.. What is the best solution? Is it worse to mark ship then refund him later? but buyer may leave negative feedback… Thanks.