I did had success contacting them through Amazon, but no answer on any other channel (not even if you use the support QR code link from the battery).
I bought two 12v 280ah battery had issue using them in series, they even sent me a free battery balancer. Measured full charge/discharge and it was 6.8kwh through inverter (with efficiency losses) from the ~7.3kwh, so I'm happy with them.