I deal mainly with BGP load sharing and use Cisco ASR and ISR routers. I’m guessing that your customer has two standard internet circuits and they aren’t doing BGP. There’s probably quite a few options, but it depends on the application. How large is the user base? Do you need a UTM device? If this is a small business, it looks like the Peplink mentioned above will load balance. Sonicwall also has options starting with the TZ300. Ubiquiti says the USGs will load balance but I’ve read it is not stable.