What are some hidden costs in transactions, payments etc. that most people are paying for but aren't aware of?

Me too. Mine was a 2.8% charge - I made bookings via Agoda and Airbnb in SGD currency on the website. But DBS mentioned that as long as it's an overseas transaction using Visa/Mastercard, I'll be charged with this fee. No issues with trip.com tho.

I had an experience for booking.com with MYR via Singapore credit card. The booking offered free cancellation. I did make cancellation eventually. So, the refund was of course given in MYR to my credit card. I actually lost about 6% due to conversion rate. I guess this is also a kind of hidden cost. Lesson learned here is to make transaction in SGD if you foresee possible to pursue refund.