According to the FAQ, it will apply on the check out page.

Did you check the expired date and status of the rebate?

So much rebate you have over there.

Weird... I never accumulated so much but the few bucks cashback from paying water bill can be used to reduce top up amount required to purchase item (first used to purchase raisin but refunded after order cancellation. Second time used to pay for pendrive). However you are right about the usage. Somehow the cashback was not used the first purchase of pendrive but worked for the second pendrive ordered from another merchant.

Maybe you can try checkout with these 2 merchants to see if it works.

1. SSL trading
2. Mono digital