Check your upstream provider and inform them of the situation regardless (some will just null-route your ip if they're not prepared/willing to deal with it), so

Check your upstream provider and inform them of the situation regardless (some will just null-route your ip if they're not prepared/willing to deal with it), some might already have protection in place to mitigate certain types of attacks. If possible, check-in with an actual DOS protection service and move your public service to a new IP behind it, whitelist only the providers IPs, free service from CF might even be sufficient. Don't pay them, there is nothing to say that they or someone else might threaten again, 10BTC is a nice chunk of change these days.