I raised it to grivance team and after a long wait they said that they updated the tap and pay offer but they made the exception that they will credit the remaining 4% within 7 days.
So far I got two transactions credited with the 4%
Best way to max that amount is to get the amazon gift balance of 10k if you use Amazon a lot. You easily cap the 500. I'm going to change the 3% to Amazon and see if the 8% works when geting vouchers on both Smartbuy and Amazon directly.