This is one of those rare online shopping cashback cards with no minimum spend. However, you shouldn’t use this card for online travel bookings as it’s excluded from the online category. Also, when buying movie tickets or fast food online, your spending could come under online shopping rather than their original spend categories, for which you will earn 2.7% for a total spend below $300 or 3% for a total spend above $300. If monthly spending is at least $300, you enjoy up to 3% savings on online shopping 2.7% + 0.3% that is if monthly card spend is at least $300.
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Unfortunately, even as Citi SMRT is a very decent card for spending, they specifically exclude travel related online merchants. That being the case, it may not favour your online spend with travel websites. You might need to get another card for travel websites.