Is in-person double-spending attacks negligible on bitcoin payments? If I'm planning to accept in-person payments in a store, is it negligible a double-spending attack or isn't it too hard to do?
Double spending attacks can certainly happen in person. A customer could have set up a system on a different computer which they remote into on their phone to initiate a double spend attack. However double spends can really only exist under certain conditions, so if you are planning on accepting unconfirmed payments, you should check that those conditions do not exist. This gives you a higher confidence in that transaction confirming. 

You should make sure that the transaction does not have:


A low transaction fee
BIP 125's Opt in RBF flag set
A locktime that is in the future
Spending from any unconfirmed transactions
Is unusually large (e.g. several kilobytes)


If you check for these things and they are not in a transaction, you can be reasonably confident that the transaction will confirm.

You can also have a system set up to immediately perform a Child-Pays-For-Parent transaction so that you can ensure that the transaction has enough transaction fee to be confirmed. Note that if the transaction is Opts into RBF, this method may not work.