First, make sure the transaction was completed for the correct amount net of monero transaction fees.
Yes, I absolutely made sure of that. Even waited for all ten confirmations.
One ~~secure~~ way to contact them is through matrix.
FTFY. And of course the (still!) embargoed clusterf*ck. Don't roll your own crypto, kids.
I also don't understand why websites are still using bespoke hand-rolled XMR payment frontends -- unless they are exchanges or (like localmonero) super-Monero-gurus... BTCPay server's Monero support is so good at this point... I have used it uncountably-many times and not once had any kind of problem.
Please folks, if you're going to accept Monero, consider using BTCPay Server.